When the crab is small you can keep any type of top feeder or mid feeder and you should be OK, however when the crab matures any fish that gets too close could become a meal. Crabs are scavengers and will eat just about anything they catch.
I've also seen my crab climb plants, filter, or heater and wait for a fish to pass and he'll attempt to jump on top of the unsuspecting fish. Haven't seen this work but he definitely tries.