Its true. They need to be in a tank with fast moving, top dwelling fish. But even that is no gurantee. They will eat feederfish without a problem. So it would be a good idea to feed sinking pellets so that they are less apt to "hunt" and more likely to scavenge. And if you do keep fish in the tank with it, don't get terribly attached to them.
By the way. If your buying it from where I think your getting it (petco) make sure that you inspect it for leaches.
i would suggest not putting in a crayfish for the simple fact that if it doesnt kill ur fish it could still def hurt your fish....they can rip fins up and even tear out the fish eyes which will in most cases lead to death depending on the shoal.....its not worth it once the crayfish sluffs it will end up a meal for almost any type of fish.....so i say no
your fish will be eaten if they are housed with a crayfish. the ones that i have kept have eaten: 3" LM bass, 4" glod gourami, and some other fish that i dont remember right now.