My 5" royal blue cray was MEAN (but awsome) he would not stang for eel shaped things. ate the head of my bicher after he killed it. bichers, dragonfish, even a farowella because it was long and skinny like an eel
I got 2 crayfish and they're w/ 2 chinese hi fins, 7 assorted goldfish. They have their caves and never bothered anyone. Everytime i feed my goldies, they would eat w/ them.