Apple snails need 2.5 gal to themselves, so you could do one apple and one betta in a 5 gal, but they do poop like absolute monsters. A nerite should be ok in a 2.5.
Edit: My apples are never slow, they might rest during the daylight, but at night they motor! Seriously slow as a snail? Whoever created that saying never met an aquatic snail.