The apple's I grabbed from petsmart have never touched my plants. They eat gunk on the floor, never seen them do too much algae work though. For algae, I can't recomend freshwater nerite snails enough. A small group will annihilate every scrap of algae (except hair algae, the long stringy stuff) in your tank and on the glass. They can't reproduce in freshwater, so you don't have to worry about infestation. Not that apple snails are a problem, mine have spawned a few times, but very few of the babies survive.
