You could possibly try White Cloud Mountain Minnows. They're classed as coldwater but can be kept in warmer waters, although the betta likes slightly warmer temperatures a compromise in the middle can usually make the two types of fish comfortable. They're usually cheap (mine were 85p each!) and I think they are soooo underated!
Snails are actually beneficial to the aquarium, they clean up algae and waste. However, its when they breed out of hand that it gets a bit uncontrolable. But the baby snails are usually eaten by the fish (this happens in my aquarium) before they get out of control. The thing is you can't always control an aquarium having snails. When I bought live plants from my local pet shop I also brought home two small snails for free! =]
They grew up and so far, I can only see those two in the aquarium. They don't come out of the aquarium either, and at most will climb at most a centimetre or two above the surface. My betta followed one of them for ages and then swallowed it and then spat it out! He stays away from the snails now!
Harlequins are beautiful, peaceful fish. I planned to get some with my betta but couldn't find them in the pet shop and the white clouds were cheaper. Maybe 7-10 or so of those in the aquarium would bring out a stunning scene! I don't really know much about corys so I cant comment on them.
Good luck with whatever you decide to do!
