The only easy answer I can give is: Probably not.
For one, many crabs require areas out of the water to be happy, so this is something to consider. But the biggest issue is that often crabs will eat just about anything they can get their claws on. That includes any fish that may wander within reach, or any fish that is slow enough and/or sleeping to get caught. Sometimes there are no problems, but its never a sure thing. Personally I'd either skip the crabs, or set up their own little tank to be safe.
\Dan