I would think the betta would attack the guppies long, bright fins.
Edit:
Are you wanting to put a betta in your 20 gallon? Because if you are, I woudn't suggest it, the danios would nip at the betta (I've tried it before)
Oh, and I know this is WAY off topic, and I'm sorry for bringing it up, but I've wanted to ask you this. Did you know that you 10 is overstocked, and I don't mean a little bit, like a lot. Koi can grow 2+ feet, the goldfish will grow anywhere from 8" up (depending on wether you have the common or the fancy goldfish) and I'm assuming the pleco you have is a common, and if it is he will reach 18+". I thought I'd just let you know, just incase you didn't. You can PM if you have any questions or would like to talk about this more (because I don't want to hijack your thread)