I've got to disagree with that last bit of info - simply based on my experience. I have three neon dwarfs in a 10g and they are fine and dandy. Just make sure you have a lot (I mean LOTS) of vegetation (real or fake)...that will cut down on each fish fighting for a territory. Gouramis aren't so much "aggressive" as they are "territorial". Also, I have two pearl gouramis in my 55g (soon to be added: one platinum gourami and maybe a gold gourami). HOWEVER, I will agree with the fact that gouramis are like male bettas in that they have LOTS of personality. You could get a really mean one, or a really laid back one. It just kind of depends.
You might try added many more hiding places if you are heartbroken about having to return it.
edit: although, some say that, like some cichlids, keeping many can cut down on aggression...either way, I'd either have one gourami, or more than two. This way the bigger one won't harass the smaller one. (thankfully, my two pearls are a male/female pair and are in love! awww)