I would also forget about the African Butterfly Fish. Corydoras are the perfect fish for a smaller tank. That being said it would seem your tank is "bottom heavy", if you know what I mean. You have an awful lot going on at the bottom but nothing at the surface (If you eliminate the ABF). A more peaceful topwater fish that can be on his own in a smaller tank would actually be a betta. I've had excellent experience with a betta and several corydoras. That being said I know almost nothing about African Dwarf Frogs.
God Bless,
Joshua