I agree with everything but the turbos, if you're speaking of the Mexican Turbo snails. Unless you've got a nutrient rich environment and resultant algae, 5 will starve in no time, though they'll get your tank squeaky clean before they do.
One turbo at most for a 12g and even then, it may eventually curl up and die for lack of food. I've had 2 die in my 25g tank after I bought them (one at a time) to clean algae. They are also severe buldozers of frags, frag plugs, clams, and just about anything else not nailed down.