When your blue gourami grows up to be 6 inches, I'd say you'll be overstocked.
Just a suggestion... mollies and platies like some salt in their water (mollies esp.). I don't know what you want to do with the gourami, but you could move the betta (fighting fish) to it's own tank (2-6 gal) with maybe a couple otos, a large snail, or a few ghost shrimp. Then get a few friends for your platy and add some salt to your 12 gal tank.