Should you add more than one gourami?
Personally, I'd have to say "probably not". Here's why:
Dwarf gouramis, in my experience, are semi-aggressive (although probably less aggressive than other gouramis). In addition, it's often difficult to distinguish between males and females. Chances are, what you'll find in pet shops are males, because they're more brightly colored. With males, having two just doesn't work, in my experience. I'd suggest having one, or at least four. And I think four would be too many for a tank of your size.
Fish are individuals, of course, so it may work. But in my expereince, two males doesn't work at all. Three doesn't work so well, either.