I have mexican red leg and scarlets in my tank. I have read of blue legs eating coralline, and since I am not rich, I do not have a lot of money to buy corals so the coralline needs to be there to look pretty.
The scarlets are not as active as the mexican red legs, but they are also less aggressive. I have lost 2-3 margherita and 1-2 nazarious snails. I am not sure if it is the crab killing them, but they are there to clean up the mess and take the shell. But, that is how it works in nature, and nature is what we are trying to replicate, so I do not feel bad.