I honestly wouldn't recommend a frog for that tank. They grow larger and are more active than most people realize/have them survive to. They also like having company.
Badis Badis grow 3-4 inches, which is very inappropriate. One Dario Dario could technically fit-though I would say they need at least a 5 gallon based on personal experience. Mine is weird, though, so this rule could not apply generally.