Note that for everything that follows, these are typical examples. Hida are typically bushi. Kuni are typically shugenja. But there is nothing stopping a Kuni from being a bushi or a Hida from being a shugenja. Often, however, such a cross-over would take the name of his "adoptive" family, so one who is born Kuni and studies in the Hida bushi dojo would take the Hida name.
Crab bushi favor heavy armor and heavy weapons. The Crab philosophy of war is to form an unbreakable shell and allow the enemy to waste his strength by crashing into it like waves against a cliff, then to muster an overwhelming counterattack and crush him with the claw. They are masters of defense and endurance, able to reserve their strength through hours of fighting before going on the offensive. They wield heavy crushing weapons because many creatures of the Shadowlands develop thick carapaces that resist the cutting edge of the katana and the piercing head of the spear. The favored weapons amongst the Crab are the tetsubo (a studded club), dai tsuchi (a sledge hammer), masakari (one-handed axe), and ono (two-handed axe, named after the "oh no!"). That said, Crab bushi are still samurai and the way of the sword is well known amongst them.
Dwarves make excellent Crab because they are resilient and it is difficult to break their lines. In fact, two of the Crab basic techniques are known as The Mountain Does Not Move and The Mountain Does Not Fall. Longtooth shifters, goliaths, half-orcs, warforged, orcs, and other resilient powerhouses are also fit for the job. The fighter and the warden, with the ability to lock enemies in place and absorb their blows, are archetypal Crab bushi.
Hiruma scouts must be fast, deadly, and unseen. They move at the head of Crab forces, tracking and observing the enemy. Their intelligence occasionally enables the Crab to forgo weathering an enemy offensive and jump right into the killing. When a battle does commence, the Hiruma scouts conceal themselves outside of the melee. Instead, they engage reinforcements, assassinate commanders, disrupt communications, and liberate resources. Once the enemy has been defeated from behind, the Crab army's immense momentum shifts from defense to offense and the Hiruma reveal themselves at the enemy flank or rear.
Elves make great Hiruma due to their increased speed and accuracy.
A small number of Crab bushi belong to the Berserkers, most of them from the Hiruma (channeling the grief and rage at the loss of their lands to the Taint) but all families are represented. These fierce warriors fight exactly as their name implies and they are used as shock troopers by the Crab.
The Falcon's Strike archers have gained prominence among the Crab since the construction of the Kaiu Wall. Freed from immediate melee combat with the Shadowlands Horde, the Crab have employed archers to thin enemy ranks (very important when goblins breed like rabbits and other horrors literally crawl up from the very ground) and cover exposed teams working outside the wall (siege engineers building traps and repairing the wall). Traditionally the Hiruma have served as the Crab's archers but with the addition of the Toritaka family and their acute sense of sight, many former Falcon have served with bow atop the Wall.
Falcon's Strike fills in that Ranger niche. The only two-weapon fighting style in the Empire belongs to a family of the Dragon Clan. Clan leaders sometimes cement alliances and trade agreements by exchanging children. Each child studies in the dojo of the other clan. Thus, the two-weapon Ranger or tempest Fighter options are available but such a character would be unusual among the Crab.
Kaiu Siege Engineers serve in the field side-by-side with their Crab brethren. They are responsible for overcoming enemy fortifications and erecting traps and defensible positions. They do so in the midst of combat, under enemy fire, and often with both hands occupied with no concern for their own safety. This dedication inspires respect and ferocity in the Crab bushi, who often double their efforts to see the Kaiu engineer safely carries out his duties.
Assembling actual siege equipment in battle is not quite D&D material. Kaiu engineers are well modeled with warlords and, especially, artificers (who can actually lay down traps and boons).
Yasuki Quartermasters oversee the proper supply and feeding of troops. They can be found in every command staff in the Crab army. While in barracks, the quartermasters seek out supplies through their network of contacts amongst the local merchants. While in the field, they must secure supplies from whatever sources possible, including hostile villages and enemy encampments. Yasuki Taskmasters are in charge of the non-samurai, or ashigeru, militia and troops. They ensure that peasant levees do not break in the face of battle and leave a flank exposed and they are often left in charge of defending remote but strategic villages from enemy advance (to deny them the top of a hill or the resources).
Yasuki are another good family for warlords or other leaders.
Kuni shugenja are the spiritual leaders of the Crab Clan. It is their duty to protect the souls of the Crab from corruption. To do so, the Kuni delve deep into the forces that fuel the Shadowlands, taking to heart the adage, "know thy enemy." Whereas the other Crab sacrifice their lives to defeat the Shadowlands, the Kuni sacrifice their souls. Their spells focus on the strength, endurance, and purity of stone and earth. They utilize earth magic to enhance their bushi brothers, wards and bindings to protect against the Taint, and the purifying power of jade to destroy the Shadowlands creatures outright.
The Kuni are your outright spellcasters. Wizards, warlocks, invokers, sorcerers, etc all blast away with magical power. Races like Dragonborn with their breath weapon are also good fits here.
Bushi characters typically call their at-will attack powers Basic Techniques, their encounter attack powers Advanced Techniques, and their daily attack powers Kata or Strikes. Kata have a real power in Rokugan and learning a particular kata is an important event. Bushi typically spend an hour in the morning going through their forms and preparing their minds and bodies for the coming day. Thanks to this preparation, when the effect of a particular kata is needed, the bushi is able to unleash the power without hesitation.
Shugenja do not differentiate their spells in such a manner. Each spell is actually a prayer to the elemental spirits, or kami, that surround, infuse, and pervade the world. When a Kuni shugenja wishes to call down fire on her enemies, a simple prayer results in a small gout of flame (Scorching Burst); an involved prayer in a fierce burst of flame (Burning Hands); and a taxing prayer in a rolling ball of fire (Flaming Sphere). The kami demand payment for their services, which is rendered in the daily prayers and rituals of the shugenja, so the more involved and taxing a prayer is upon the kami, the less likely they will repeat their performance.
All acceptable magic in the Empire is technically divine. Shugenja are the priests of the kami (the elemental spirits) and the sole spellcasters. They cast their spells through prayers to the elemental spirits and these are nature spirits that are nameless and ubiquitous throughout the Empire (not as common in the Shadowlands as many have been warped into or replaced by kansen, evil elemental spirits). There are five elements: Air, Earth, Fire, Water, and Void (void is ostensibly the absence of any of the other elements and yet it appears when you combine the other elements).
Pantheon: There are gods in Rokugan (Lord Moon, Lady Sun, the celestial Dragons, the countless Fortunes who rule over aspects of daily life) but they do not have dedicated priests/shugenja. All shugenja know the rituals appropriate to each Fortune and perform them for the people (crop-growing ritual for Fortune of Agriculture, marriage ritual for the Fortune of Love). The most important Fortune in Rokugan is the Fortune of Rice; the most revered among the Crab are the Fortunes of Strength (Bishamon) and the Fortune of Fire & Thunder (Osano-Wo, the son of the kami Hida). However, the Fortunes are worshiped as a whole body; they have individual temples throughout the Empire but no shugenja would serve only one and ignore the others.
That brings us to the Brotherhood of Shinsei. These monks serve in the temples of the various fortunes throughout the Empire. Because they serve in temples dedicated to a particular fortune, they place a much heavier emphasis on that one fortune than any shugenja would dare. They do not, however, ignore the other fortunes (that would be blasphemy). There are several Orders of monks within the Brotherhood of Shinsei. The Order of Osano-Wo, Fortune of Fire & Thunder, is both the largest in Crab lands and the most militant. Its monks are sohei, or warrior monks, who see themselves as the ultimate expression of the soul's trial through the mortal world. They toughen their flesh just as they harden their spirit to resist temptation and corruption. This is in contrast to most other monk orders who focus solely on the mind.
The flavor text of your abilities is as mutable as using any race to model a human. A cleric might describe Divine Glow as a burst of fire rather than light (it is easy for the fire kami to exclude one's allies from the fire). I think I mentioned the iaijutsu duelist (drow rogue) who describes Darkfire as his intense concentration and tapping into the Void. Rules text is rules text and flavor text is flavor text. Feel free to change the latter as needed.
In answer to a requests for help on a character's background:
Briyu could have grown up among the Dragon Clan. Before he was born, the Crab and the Crane were at war (they almost always are). However, the Crane had managed to secure the aid of the Lion Clan through a series of political favors. In order to match the strength of the Lion reinforcements, the Crab would have to pull troops from the Wall. Instead, the Yasuki negotiated for the Dragon Clan to amass forces on the Lion border and thereby split their attention. In exchange for provoking the Lion Clan, the Crab agreed to lend the Dragon the services of several Hiruma scouts. Briyu's father was among the Hiruma who were sent to serve the Dragon, both during the skirmishes to draw the Lion's attention and for several years after in payment. As a result, Briyu grew up in Dragon lands and studied with the Mirumoto family, the originators and practitioners of Niten, or two-sword style.
Typically, playing a foreigner in Rokugan is suicide. In 440, a group of gaijin (foreigners) is allowed to conduct trade and is given access to the imperial court for 2 years to prove their honor. At the end of the 2 years, they are commanded to leave the Empire. They boarded their ships and left only to return soon after and open fire on the Imperial Palace with their cannons. The opening volleys took the life of the Empress. It is a bit of a sore spot for Rokugan and gaijin were forbidden to enter the Empire.
That said, individual gaijin are not unknown in the Empire half a millennium later. The best bet for going this route would be the completion of a Twenty-Goblin Winter. When the Crab Clan has a particularly bad year and the losses on the Wall are greater than they can sustain, the Clan Champion will declare a Twenty-Goblin Winter. Anyone who enters the Shadowlands and returns with the heads of twenty goblins (or other foes) by the end of winter is allowed to join the Clan. Typically this is a route used by desperate ronin to regain the perks of fealty. If a gaijin were allowed to join the clan this way, he would definitely be kept near the Wall and often sent into the Shadowlands. The Crab are already the least adept clan at courtly politics and allowing word of a gaijin Crab to spread would be disastrous.