Three panda or pygmy cories are a valid option, or one otocinclus... however all three of these fish require very nice, stable, clean water conditions in order to thrive, as they are rather delicate.
Apple Snails reproduce sexually, and need about 2g per snail (imho).
I can't think of very many fish that would work in a 5g with a betta, other than that. 5g options are already rather limited, when you add a carnivorous fish to the mix, all the mouth sized fish dissapear from that list (literally heh).