FWIW, I did keep a 10g tank with a rather substantial blue crayfish (probably 5" long) and some cold(er) water killifish for 6+ months and never lost a killi to the cray.
I think the rule of thumb that should guide your thinking is to be prepared to lose anything else you put in the tank with the cray--including not only fish, but other crustaceans (ghost shrimp, snails) and any live plants. Some things may survive, but don't put anything in there that you aren't prepared to call crayfish food.
I did keep a really, really large apple snail in the tank okay. Every so often the cray would grab the thing and try his best to get in the shell to get the snail, but the snail would be able to retreat far enough he was safe. After 10 minutes of frustration the cray would just give up and walk away, and go about his business. This was with a snail about the size of a golf ball or a little larger. The smaller one I put in there was lunch within the first 24 hours.
And you probably already know this, but there can be *no* openings in the tank at all. I finally lost my cray because he got out one night and made it out the door of the room and 30 feet down the hall, and dried out before I woke up in the morning and found him. No HOB filters, no nothing, you need an almost perfect seal between lid and tank.