If you only have 1 snail, it's going to take a very long time for him to eat all the snails. In fact, he probably won't be able to keep up. The only problem with getting more is they will breed(though not as readily as pest snails). I personally don't see this as a problem though, as they are lovely looking snails. Plus they don't eat plants! Once all your snails are gone, you can still keep them as they will eat fish food too.
Have fun
Don't know all your stock but back when I kept community I found Zebra Loach were a great pest snail eradicator! Darn good looking fish too