Crayfish
I hate to be the one going against the more experienced fishkeepers, but my crayfish hasn't done much damage to my fish yet, and I've had him in my 55g for several months. He lives peacefully with 3 cichlids, and oscar, and some sharks. I think that as long as you give him a good hiding place (i usually make little caves with rocks for him), he won't bug other fish.
The only fish I ever saw my crayfish kill was a small pleco (about 1.5"), and that was because the pleco was sick to begin with. I think that a pack of crayfish will attack healthy fish and bring them down, but one crayfish by himself wouldn't have the balls to bring down a fish that could defend himself.
That's just my experience, and I honestly wouldn't consider myself an experienced fishkeeper, so I don't know how much weight you want to put on my opinion.