I've never owned crabs or crayfish as pets before, but I've had success with Mosquito rasboras and White Cloud Mountain minnows in a community tank with shrimp and snails. They are very pretty schooling fish and fairly tolerant to other species. The only thing I would be concerned about is the crayfish or crab feeding on the smaller schooling fish. I'll leave that to your discretion.
Additionally, there are a few tetras such as Cardinal/neon tetras and rummynose tetras that are reputedly good schooling fish for shrimp and snails, though I've had no such combination personally. (source)
Concerning your crayfish/crab combination, I really wouldn't advise it because if you scroll through forums there are plenty of reports of crayfish and crabs killing fish and feeding off the dead bodies (who can blame them?)
 - Just be careful with raboras, because in their natural habitat, they feed on very small crustaceans. I wouldn't be too worried about it if your shrimp/crab/crayfish are bigger than the opening of their mouth.