If you're talking about putting all three of them in the 20gal in your sig, I'd say it highly depends on how many other fish are already in there and how densely planted the tank is. Gouramis are very territorial and, although they seem to have varying personalities, some of them can be downright nasty. Although not written in stone by any means, it would not be out of the ordinary for a mid-sized gourami to claim most of the 20gal space as his own territory. This could lead to fatal turf wars. Sadly, this is not information I received in a book or from helpful staff at a lfs... it's information I base on my own trials and tribulations with gouramis in my first year of fish keeping. It all comes down to terrritory - if gouramis feel they have adequate territory, they are quite peaceful fish and very tolerant of one another. If not, they can and will kill one another and other fish. Just my humble opinion.