First off? How old is your tank? If it is already established then I would say a few more turbos would be nice. Then, add some hermit crabs(both red and blue) and then a cleaner shrimp for starters.
if I had to add more snails I would get a variety maybe a queen conch or fighting conch and cerith snails. crabs emerald, sand sifting crab, or a sally lightfoot crab.
A variety of cleaners is always a good thing.
Mike
astrea snails are also a hardy little snail.