I have Bubbles, the blue crayfish. He lives in the 5 g with an increasing number of minnows. He hides during the day in his skull shaped home, and fishes during the night. I just don't watch!
He is really cool looking and doesn't require a lot of care (PWC once a week, food once every other day, a filter and no heater).
My daughter had another one in a 10 g that made a number on a new batch of tiger barbs. He just chew them up!!! After that he was kept alone until he left for cray fish heaven. :-(
I recommend you keep him isolated and if you want to be really nice to him, give him "room mates" you do not care much for!