You should have hiding places for the crayfish where other fish can't attack them, and where they will feel safe. Sorry about this last bit of info, but you can only keep 1 crayfish to a tank without the 2 crayfish fighting, and resulting in a lost claw, or leg. If you really wanted to, get a tank divider, and then you can get 2 crayfish. Try shrimp that are good together, like bumblebee shrimp, or a bamboo shrimp.