in my opinoin and experience crabs are best kept to a species tank. You may be able get away with having them with bigger fish but then the fish are likely to do the damage to the crab.
or you very selectively pick there tank mates which stay at the top but even then you risking the fishes live.
I have a Red Crab and a Pictus Catfish, ( Im not sure if there bottomers but mine hangs around on the bottom and rarely goes up top). I have found they go very well together. It might just be a one off. If it is then im very lucky.