I was thinking about this too.
I'm thinking its going to be more of a trial-and-error situation. I've had my danios for almost a year now and thier tails have grown so much, they're almost like a flowing guppy tail or something.
I'm not sure if the danios would go for the bettas fins, or vice versa. The danios are very fast though, so if the betta were to go for them, it'd be tough for him to catch them.