First, what size tank is this? Even the smallest species is going to need at least a 75g....
As far as what goes in it, well, rocks and sand. You need enough rocks to make the fish feel at home, give it sleeping spots, and to provide biological filtration. They like sand because it's natural for them to dig around in it. Keeping the sand clean probably won't be a problem. Keeping the tank clean is going to be more up to you than anything else. Like you said they will go after hermits and other crabs, starfish and urchins are on the menu too and they "might" go after snails too (depending on size and type). Now, I've seen some tanks that had the large red starry hermits in there with large triggers and that seemed to be working but be warned that the large hermits are quite capable of attacking and killing small fish.
In my own tank, I have a small picasso trigger. Right now there is a large turbo snail in there but that may or may not last. I did have two very small hermits too, but I haven't seen them lately. I do have a nice patch of feather calupera along with some shoal grass. I'm hoping to eventually have a good amount of macros and marine plants to help keep the water clean as the trigger grows.
Oh and I do have a filter on this tank plus a decent skimmer!
HTH
