ADFs are like passive, little, blind mice.

I've seen blood worms land on their head and have them not notice. They see movement better than still things, and they have a good sense of smell. If they snap at fish, it's usually because they caught sight of the fishes tail and went, "AH FOOD!". Also, they have no teeth, so having a frog suddenly latch onto their tail is the equivilent of being suddenly gummed.

I keep 3, I think they rock. They're massively cute, as cute as they are stupid.
I'd recommend avoiding the apple snail. Oh, you can keep him. But they're rather messy guys, and sometimes the bettas take a fancy to their feelers and attempt to eat them.
The African Dwarf Frogs, though, would work, wouldn't go more than 2-3 for bioload's sake.
Just keep in mind that there are African
Clawed Frogs out there, that get to be 8" big and eat fish, and sometimes petshops either a) don't know they get that big, or B) think they're ADFs and mismark them.