I put snails in and a few hermits, acclimatised them and let the snails go first. I added the hermits after another 1/2 hr, this was to let the snails hide before the crabs went in.
Just get a couple here and there. If you put to many in at once they'll run out of food to fast. You want them to gradually graze it down. Basically find a balance.
Most commonly stressed snails are:
Turbo-can get big and knock over small rocks or corals but they do a good job
Astraea-not tropical snails. Migrate there but in tropical waters their life span shortens. Do a good job.
Cerith
Nassarius-eat detritus and junk in the sand. they'll also climb rock work occassionaly.
There's a few. Read about them before picking them up. Don't always trust a lfs for accurate information.