How big is the crayfish, and what kind of cichlids are you talking about? Oscars seem to eat almost any kind of crayfish very quickly. If you do put them together, make sure you have plenty of hiding spots the cray can get in, especially when it comes time for it to moult. A good test I'll sometimes do is lower a crayfish in a net to the bottom of the tank, and see how aggressive the fish are towards it. I have 2 Orconectes Propinquus crays in a tank with a bunch of bluegill and largemouth bass, and its worked out fine.
