For such a small tank, I'd avoid the turbos. They can get really REALLY big and are like bulldozers in your tank. If it isn't glued down, they'll knock it over. They're great algae eaters... but they're just big bumbling giants.
My favorties for rock/glass are trochus and astrea. Nassarius are great for the sand bed. You could easily pop 10 nassarius in there, and then spread the other 8-10 snails out between the trochus and astrea. Trochus are great because they're a little more acrobatic when they get flipped over and can many times right themselves, unlike the astreas.