Is this a good combination of fish for a 20 gallon tank?

The betta and the guppies will end in sadness. A betta will attack other bettas or other fish that they mistake for bettas (guppies fall in the category usually). I'd get rid of the betta, make sure I have 3 female guppies and one male guppy, and then, when breeding occurs, the other fish will feed on the fry leaving the other fish alone (may sound gruesome, but it happens out in the animal kingdom, and it'll happen in your tanks too). Hope this helps.