Hermits won't tear apart the live rock, some will pick on certain corals though. Personally, I like the small blue legged hermits.....they don't have the reputation of killing snails for their shells, and the ones I have don't seem to bother anything.....
As far as your sand, conchs are good....fighting or queen conchs (although I forgot to mention in another thread that queen conchs will eventually outgrow just about any tank and must be traded in when they get too large for your tank). Most of the critters that keep your sand clean, like various worms, pods, and small brittlestars will arrive on your live rock and seed the sand, or you may have to buy a starter kit or get some sand from an established tank.