Becareful with blue legs, they will kill the snails for thier shells. I have three Emerald crabs with various hermit crabs. When the weather clears up down here in Florida I will go out and get a couble sally light foots to help keep the live rock clean.
Do some research on the web and see what is there for what you are looking for :wink:
Actually, hermit crab is more of a shrimp than a true crab...
I can't think of any reef-safe crab. Some Acropora crab that hitchhiked in coral colony can be consider reef-safe, but they do some damage to the coral once a while.