Personally, I would not put any algae eaters in a 5g tank, esp. a divided one. Though some species are small enough to inhabit such a tank, all of the ones I can think of are social and would be miserable without others of thier kind. Since two bettas is allready about as stocked as a 5g can get, adding a trio of ottos or something of the like would be out of the question.
As for snails... eh. My personal experience has been that bettas have a tendancy to chew on them, and the snails themselves make a lot of poo. However, many other people here have reported sucess. So, let me ask this. Have you any filtration for the tank? If not, I'd say no snails; they are turd factories and will damage the water quality. However, if you have decent filtration, you might be able to get a small apple snail.