I used to have a regular brown crayfish(blues are 20$) in a 15g. His name was dr. pinchy, he was really cool. I had some zebra danios in with him to see if he would catch any. He never did but they sure got him out of his cave, he was always out stalking them.
I fed him shrimp pellets,any sinking pellets, veggies, flake food, pieces of chicken and shrimp, worms.Basically anyhtign will do.I also added clipping from my planted tnak as I loved to watch him shred them up.
I seemed to have a good one as he didn't kill fish or kill the plants i put in(he shreded clipping but not the plants in his tank?weird). Some crayfish can't be kept with anything or usually hide all the time or esacpe from the tank.
If you have the room they are great pets, I would get one again.
ps: the only thign i had trouble with was temp, some species like cooler temps and some like tropical temps. Except at teh LFS they dont' tell you what species you have. I just kept mine around 75degrees