I used to feed my breeding pair of Oscars crayfish regularily and they really enjoyed them. Like one other poster said before, you have to be sure they can fit in the fish's mouth. 3-6 inches sounds too large for juvenile though. You have to remember, the fish *will* TRY to eat them, no matter the size. You could end up in a situation (as I did once) where the fish swims around for ages with the crayfish hanging out of it's mouth, trying to get it down. This is *not* pretty, and quite frankly, dangerous to the fish's health. (imagine those claws swinging around next to the fish's head.......) Wait until your fish are larger, or you find smaller crayfish. They'll appreciate the occasional crayfish in their diet then.
I have no idea. Perhaps a google search might come up with their preferences.
Do you mean to let them live in the tank with the fish? I've never done that. I kept the crayfish in a bucket with an airstone and fed them directly to my oscars. I usually bought a dozen at a time and fed them over the course of a week or so. Did this about once every month/6 weeks. I always worried that the crayfish might bring "hitchhikers" into my tank and was very careful dealing with them.
