It is best to have a mix, and only add a few at a time, that way you'll be able to see what kind of snail you still need to buy, and won't starve them.
Some of the best snails are astreas, trochus grazers, and turbos. The turbos get pretty big, and they can move rocks sometimes. Stay away for Mexican turbos because they live in colder temperatures. Asteas will usually die if they fall over because they can't right themself, so if you see one flipped over, help it up. They don't fall over too often, IME. Cerith snails are pretty good, too, mine are usually cleaning the glass, hermit crabs seem to like their shells but I haven't had a problem with them yet. Margarita snails like cooler temperatures, too, so they'll die in our tanks. Nerite snails seem pretty good too, but I don't think they're as efficient as the other snails. Nassarius snails are awesome, they burrow in the sand and as soon as you put food in the tank they come out.
Hermit crabs are good scavengers, they eat left over food and detrius and some kinds of algae. Some kinds will eat snails and use their shells. Blue legs are known for that. Scarlet hermits are usually peaceful, and don't get too big. I find the yellow tips to be peaceful as well.
Starfish usually aren't good in a nano, because if they die, they decompose very quickly and can cause a mini cycle. Some are good to have, others will eat fish and corals.
Shrimp are generally good to have, with the exception of manits shrimp. Cleaner shrimp will sometimes clean parasites off other fish, and some kinds of shrimp will eat algae or aptasia.
That pretty much sums up most of the common stuff... hope it helps.