I have two triggers and a eel and so far nothing in the tank has eating the snails or the crabs. But i make shure i put large snails and crabs in the tank and keep everything well fed. also have a green serpent star.
i would think that if you got the larger crabs and snails that they be able to fend for themselves.. it's worth giving a try at least! just keep the trigger well fed and it will hopefully leave them alone!