what i konw about crayfish whatever they can catch they eat so you need big fish and they are the masters of escaping your tank so they need a very tight lid
crayfish are species only, they will eat small fish, and large fish will eat them (especially when they are moulting). They are invetebrates not fish, they are interesting and fun to keep, but you need to have a seperate tank for them.
Crayfish are best kept in a species tank it is true. What I do in many of my tanks is buy feeder guppies. They have babies all the time, makes the tank much more active. Its actually rare that any get eaten, but it can and does happen from time to time. Feeder rosies or goldfish are also fun to get with them. I figure I'm giving them a better and longer life than if they stay at the LFS and get fed to oscars, turtles, etc.
oh yeah remember not to put cray fish into a planted tank,
mine have the habit of uprooting the plants (Egeria densa, Hydrocotyle verticillata, Blyxa echinosperma) and eating them although i feed it with pallets and carrot.
it depends on what crayfish you have. i recommend you check out www.bluecrayfish.com
it is a very informative site with a Forum and some really nice people.
good luck with yours they are great pets, i have 4 cherax quads. i got them about 5 months ago and i have sucessfully hatched two batches of babies already.