Pumpkinseed and redear sunfish, both also known as "shellcrackers", are good snail eaters and have water preferences similar to those of gar. Dojos are slim enough to be easy gar snacks, but a large clown loach would be safe from most gar (I'm guessing you have a Florida or other smaller gar). The trouble with clown loaches is that they need warm temps (high 70s to low 80s).
If you have room for more fish in the tank and want to add some anyways, get one of the ones above. However, if you are only concerned with getting rid of snails, I suggest you do not get fish to clear up the snail problem. Either assassin snails as LP mentioned or manual removal would work. If you put in a piece of lettuce or cucumber, snails will congregate on it and you can easily remove them. A few rounds of this and you should be snail-free.