Heh. For their size, male bettas have some of the strongest jaws in the animal kingdom, and they have sharp teeth too. Most of them are not aggressive to anything but other bettas, but every once in awhile you get one that wants to take a chunk out of anything else living that gets near. While they would certainly not pose any danger to adult frogs, if the adults deposit tads in the water feature, a male betta might harrass them. If you do decide to get bettas, get female bettas. They are social (so can be kept more than one to a tank) and non-aggressive.
I used to have a male betta that would attack any other fish in the aquarium. He would even flare out at fish that looked nothing like another male betta, such as catfish and alge eaters. I ended up having to move him into his own tank. I didn't want to get rid of him, because he was the most gorgeous betta I'd ever laid eyes on, bright wine red with comb fins. But I couldn't keep him with anything else.
I used to have a male betta that would attack any other fish in the aquarium. He would even flare out at fish that looked nothing like another male betta, such as catfish and alge eaters. I ended up having to move him into his own tank. I didn't want to get rid of him, because he was the most gorgeous betta I'd ever laid eyes on, bright wine red with comb fins. But I couldn't keep him with anything else.
