I do about 1/2-3/4 of a snail per gallon. Obvi something bigger like a serpent star would count for a couple snails. A lot of people like crabs, they run around a lot and look cute. But for me that many legs is creepy, and I don't like how they kill snails. Some colors are supposed to be more tame, idk which though.
RN in my 55 I have 10 dwarf cerith, 5 cerith, 3 fuzzy chiton, 10 trochus, 5 nerites, 10 nassarius, a fighting conch and a few limpets, periwinkles, and stomatella. It's a pretty good mix imo