My emerald crabs grew huge in the last year an a half, some of them are over 2" big at the moment, with huge claws, however i never seen them pick at anything beside green algae
i once had a starfish with a taste for fish, i had this long nose butterfly that he took care of once it was injured... but like i said i never had any problems with those emerald crabs even those they could easily catch fish, and sometimes when the fish bother them they stand on their legs and open their claws, but i never seen them attack anything