Actually, I've found dwarf frogs to be pretty slow. They're actually terrible swimmers. But they do great with ambushes to make up for it. Lie in wait, and then a quick strike. But they really aren't very fast swimmers. Sometimes they can propel themselves upwards pretty fast if they have something to push off of, but when you watch them swim normally about the tank, they look like slugs. Well, faster than slugs, but slow enough as far as frogs go. Also very clumsy and so not great for chasing down prey.
The main fear I have is actually the fact that the frogs are fairly delicate. A bite from a betta may break a bone. If a frog gets a hold of a fish that size, my biggest worry would be a bit of fin damage from the fish trying to escape. But they only really go for the fish if they're being badgered, as you saw in the videos. xD
Well, they're getting along today. The betta seems to be...well, scared of them. They blend in with the sand, but when they move he see's them and flares. But aside from the food fighting in the videos, nobody has bothered the other. I think it's more of an "AH WHAT'S THAT...oh, frog...-swims off-" kinda thing. But I'm being watchful anyways.
I have another tank he can go in, but the one he's in is set up for my betta and frogs. I have it -almost- the way I want it, though I would like to add in some more taller plants once mine grow bigger in the other tanks, and also I was thinking maybe a betta hide. You know, those little nest like things that float up, he doesn't seem to like my cave like the female did. lol I may remove that and get him something he actually likes, something off the ground.