Kroot Trait Ideas (Based off of Wikipedia entry):
Carnivorous: WP or something like that to try to force themselves to eat anything other than fresh meat.
Scaly Hide: Small armor bonus, potentially combined with Agi loss.
Blindsight or Thermal Sight: +/- to Per, depending on what is being done (Tracking enemies gets a bonus, reading text gets a penalty, or might even be impossible, etc).
Efficient Digestion: Kroot don't urinate or defecate, apparently, so something to do with how much energy they can get from consumption, maybe relating to how much rest they need compared to other races.
Pheremones: Probably adjusts Fel, depending on the Kroot's opinion of someone.
Rapid Mutation: Increases the amount on the die roll for Mutations, or perhaps lets the Kroot player modify it by a small amount, if they wish.
Tribal History: Modifies how they interact with other Kroot, or just gives them some basic starting proficiencies.
They should probably also have higher S, T, and perhaps some size benefits for being the size of a Space Marine (Aprox. 7 ft, minimum.)