Ok folks, I'm a crayfish kind of guy and have an answer for you. First, you need to know exactly what cray you have. Is it north american or from the A.U.? If it is american, it is likely Clarkii and may cause issues with certain fish. If it is Alleni they are slightly less aggressive with fish. Obviously you stay away from bottom dwellers like corys or long finned fish like bettas. The faster it moves and further from the bottom it swims the better. If it is an A.U. crayfish-Cherax-the same general rules apply only the claws are so large that they can't move fast enough to catch a fish. All three listed above have the large claw problems, Cherax take that to the extreme.
In my experince, tetras, gouramis, livebearers and most barbs are safe to add. It helps if the tank has some height to it as well. I have crayfish in all but four tanks, that means fourteen tanks have crayfish. They are in with Endlers, Least Killies,guppys, goldfish, Pike Livebearers, killie fish and tetras. The only time, and it is rare, that fish get in trouble is when they are sick or old. Crayfish like to wave their claws around and look tough, they are not.
