I wouldn't worry much about snails, because they (with few exceptions) do not do any harm, and since you already found one hitchhiker, there is almost a 100% chance that you missed others and that they already got into your tank, so I would just add it to the tank. I myself have thousands of snails (Malaysian, one red rounded one that I don't know the name of, and the fast moving kind) and so far they weren't a problem, they mostly eat algae, detritus, fish food, and rotting matter and I just leave them alone. I think that they add interest to the tank by providing more life and movement to it and I don't mind them cleaning all the waste. I agree that they multiply very fast, but I also agree that they can't multiply past the amount of food available (that's the first thing you learn in a biology class), and in the case with the fast moving snails, they get so big that you can easily pick them out with your hands...
