What kind of snails are you thinking of getting? Most snails are scavengers and not very picky eaters. They will eat just about any flake, pellet, or fresh veggie you put in the tank. If you have live plants, they may choose to munch on them, though.

Snails generate hardly any waste, so they really won't have much effect on your bioload.
This is a great site about apple snails.
http://www.applesnail.net/