If you are going to quarantine an invert you'll need a cycled, mature tank as they graze on surface film constantly. I do recommend QT-ing especially if you got it from a sketchy store. With an apple snail, nothing smaller than a 5 gal as they are surprisingly good at producing copious amounts of ammonia.
As for a snail infestation, you won't get one if you don't overfeed. Infestations don't just happen, they are made. If you see too many snails in your tank then something is wrong. If your apple snail does bring in eggs of another snail, you will be able to see them and can remove them. If you wait until they are adults, then you'll be waiting at least a month. Snails are hard to spot when they first hatch.