At one of my LFS, there is a tank with about 10 6-7'' crayfish. Would they get eaten by a 6'' severum or 3'' parrot cichlid. Also I have some tb's but I don't think they'll get eaten cause they never go lower than 6'' of the bottom, plus they're way faster.

It could be a rusty crayfish.

Cray fish have a nasty habit of going after fish at night when they are sleeping on the bottom of the tank. As for the larger fish I can't answer if they would work, maybe someone else has experience. The exact species of cray is important as some get very large. Crayfish do need a cave or other place to get out of the reach of fish so they can shed their shell in peace. Only one cray for tank is usually a good idea.