The applesnails that you can buy in the store won't do a thing for algea...most would rather starve than eat algea (at least thats how mine are). They breed quite a bit if conditions are right and you have a male and female, but their eggs are laid in clutches above the water so its super easy to remove unwanted eggs...so no problems with uncontrolled breeding.
With apples you need to make sure there is plenty of space for them, they're poo factories. Almost as bad as goldies. Unless you want them because you really like them I'd recommend passing because all they do is muck up the tank and look awesome. They need at minimum 2.5 gallons per snail, depending on the variety. I'm assuming you'd be getting brigs because they're the only ones readily available because of shipping laws. My canas and haustrums get 5 gallons per snail because they're so much bigger than brigs...but you should plan on 3-5 gallons per snail for them to be comfy.
As Crissy mentioned snails are piggys, you'll have to get them snail biscuits (or make them yourself) and or give them fresh veggies on a regular basis. Left over fish food isn't enough. My snails love dandelion leaves and romaine lettuce.
If you want to have access to everything you need to know and more about apples you should check out www.applesnail.net . They're the local snail experts. Hope to see ya over there