You could put four in there, but only one male. The males usually fight. The females can be pretty drab, but I did see some nice looking Powder Blue females on this forum before. I'm assuming this is a standard rectangular tank.
If you mean Powder Blue and normal the difference is just color. It's the same species. I'm not so good at describing color, but if you google Colisa lalia then click images, you will see them side by side. The Powder Blues are the metallic blue looking ones. The standard male Dwarf has more red striping. There are two unrelated species known as Platinum Gourami. One is a color version of the Three Spot Gourami, the other is also known as a Moonlight Gourami. Both are inappropriate for your size tank.
