Power crawl sounds neat!
Skrell should have maybe some sorta hyper sea creature toxin, maybe they can get a stinger implant / gland / whatever. Paralysis or who knows what! Maybe give it a few different modes!
Plasmamen..... maybe some relic from where they were located? Spooky purple skeletons they are... like... a Plasma Sword. Looks like a purple sword with runes, its flaming, does some some solid brute and minor burn damage, give its a nice parry chance and the ability to sometimes block projectiles. Make it very expensive.
Humans should get some odd options too! Cooky cultural things perhaps. Not really sure what.
All in all I really like the concept! Creates a cool dynamic for traitors and unique species choice.