i've kept Hallowe'en hermits, but ANY hermit will eventually get around to snail killing. hermits are opportunistic omnivores, and snails fall right into that category (the "meat" of "meat and taters"), esp. the species that can't right themselves (e.g., Astrea). the thing is, hermits don't necessarily want the snail's shell, they often simply have a taste for snail meat, esp. if the pickings in the tank are slim.
the only hermits i keep these days (if i keep them at all) are the scarlet reef hermits (red w/yellow eye stalks), as they seem to be the most docile and least troublesome.
two hermits is plenty for a 30 gal tank, esp. a new setup.
HTH