Well, the general rule is one snail or crab for each gallon of water. If you do that, they will keep the tank balanced pretty perfectly (assuming the water quality is where it should be). As far as the tang goes, you should be supplimenting his diet with dried sea weed or Nori a few times a week anyway. If you do that, you wont have anything to worry about.
If you have questions about which snail to get, check this out..it will give you a very good idea of what each type of snail looks like and what its habits are.