I have a Hammer's blue cobalt crayfish (the more aggressive one with the bigger pinchers) and I keep him in a tank with African Cichlids (all medium sized, about 4" now and won't grow over 5") and other aggressive fish with no problem. They've all been together a long time and he's about 4.5" right now and he's even shed a couple of times in the tank with those aggressive bullies without a problem. They seem afraid of him for the most part. I can tell my crayfish is aggressive, he's always quick to get his pinchers up, and I have seen him shoo fish out of his way without ever hitting or hurting them, but he knows he can't catch any of the fish in my tank, and they know he'll rough them up in no time with those pinchers (it seems like he has problems walking because of their size, they're just that big).
The best part is that he's best friends with my Tiger Botia Loach (he's about 6"), they hang out together all the time, they share the same piece of driftwood as their home (or rock if one of them decides to move for a moment) and even seem to watch out for each other. I've never seen any sign of fighting between the two, in fact they act more like the police in the tank, breaking up the Cichlids as soon as they begin to display any sort of aggression. Everyone that sees them is amazed since Loaches are the ultimate bottom dwellers, maybe since they're both nocturnal that helps them get along. But in my opinion, just like people, they all have different personalities.
Only thing I regret is that I can't get a puffer for my tank (tank has enough room, and the South American puffer has caught my eye) but I'm afraid that he will in fact go after my crayfish. Only thing that might save me is the fact that the crayfish is already bigger than the size the puffer will get in captivity. Anyone have any thoughts on this? Maybe if i make sure the puffer is well fed on a diet of snails and such?
Either way, Crayfish are really fun creatures to have and watch, constantly exploring and climbing things (at night anyway, mostly he just hangs out under his driftwood during the daytime). He's always a conversation piece whenever somebody new sees him. I recommend them to anyone that has fast enough moving fish, or if you're afraid he'll rough up your fish get him his own tank, you won't regret it.