I have 2 ADFs with my betta in about the same size aquarium. They are doing well and generally leave each other alone. Last feeding my betta, Peter, ended up at the opposite end of a bloodworm from one of my frogs. They worked it out without a lot of fuss. I love my frogs :) I haven't had them for very long, so I might not be the best person to get advice from. I don't have any regrets at this point. I do believe they are social and like to be around their own kind. I think the key is making sure that everybody is eating enough.
Peter - Male Veiltail
Pinky & The Brain - African Dwarf Frogs