WOW- Where has this thread been? Hehe...
Jon- I would say it is the percs that are picking on it. I have tons of emeralds and They dont pick on fish. I have proof of this. Last night, I put all the fish and crabs into a bucket. M emeralds were all hunkered down in a corner together and as I went to get the fish out, All of my fish proceeded to land on the pile of crabs. The crabs didn't really move, or claw the fish or anything. I think the two crabs you have are just fine for a 20gal. The only concern is starvation. My crabs eat algae, and also meaty pellets.
I would go with Nissans plan of removing the percs, and moving the rocks. You can also try removing the percs for a day, and then put the fire fish in. Then when the lights are off, you can put the clowns back in, and that could help by confusing the percs.
Let us know!