Go with Honey Dwarf, Sparkling, or Sunset Thicklips. They are the most non-aggressive peaceful gouramis I know. They also remain fairly small, about 4" max.
Every gourami is an individual, that is something you must watch out for. Some people get lucky and get a very submissive "friendly" gourami, other people get small terrors! Doesn't matter male or female, both can be very aggressive and destroy tankmates on a whim. I would stay away from gouramis that grow large, such as pink kissers, gold, blues, opalines, snakeskin, three-spots, moonlights, leeris, and pearl. They will outgrow a 20 gallon tank, and the larger they get usually the more aggressive they become.
Usually with gouramis it is recommended to have two females and a male of appoximately all the same size. This does not mean less aggression, it just means that the trio would be so busy chasing each other they won't have time for chasing tankmates. Fish like to set up a pecking order, and will continue to be aggressive until they figure out who is top fish and who is bottom fish. Most gouramis have mating aggression as well. If the male is horney, he will harass the females. If the female doesn't want to, she'll beat the crap out of the male. You don't have the room in a 20 gallon tank for a trio of gouramis, especially of the larger grown species.
Stick with dwarf species. Sometimes *knock on wood* you get a real docile single male or female, sometimes you don't.
~~Colesea