Well, your 20 is very stocked as it is, so I wouldn't put it in there. The angels alone will account for all of the bio mass the tank can handle, and I'd very strongly recommend getting rid of the rest of them. The pleco will get far too large for a 20. What kind of gourami and tetras are they? They should go to, if not to just reduce the amount of fish in your tank. Otherwise, if you were to upgrade, those fish are generally fine with bettas with the following exceptions: Larger gouramis, such as blues (and all of the variations, like opaline and gold) do not get along well with bettas and the tetras, again depending on species, may nip the betta's fins to nothing. Something to watch out for.