My wife has a betta in a 2.5 and is looking for a "tank mate" for him. She tried both a snail and a pleco and the betta bullied both mercilessly, so he's all alone again. I've seen a lot of people have had luck keeping frogs with them, but I was wondering what other options are out there.
Other than a snail you really don't have room for anything else. If you upgraded to a 5 or a 10 you could certainly keep a frog. You may even find that your betta is less aggressive towards other fish in a larger tank.
Actually, a 2.5g is fine for a frog if it's heated. But frogs are really defenseless and if the betta picks on snails and plecos, the frog won't have a chance and will get really beaten up. If he won't accept a snail, he's probably a loner for life.
I am by no means an expert in this area, but I have been told by moderators and super fish, etc. that an ADF can be easily kept in a smaller thank with a betta - I was even told that I could get one for my betta, and he's in a 1.5gal tank. So, it probably just depends on water change dedication, etc...