Since you are getting a fish only, and assuming that your fish are all gonna be "snail safe", if your tank is already established, I'd add about 25 snails. If it's a new tank, I'd add about 10 snails, then more later. Regular turbos, not mexican turbos.
If you have your LR then an emerald crab is always beneficial, and if you wanna do hermits, the scarlet ones are nice. Just remember to get extra shells. They are good for food that drops to the bottum of the tank. Even if you get 5, that would be enough.