You could do at least 6. It is always risky to put dwarfs and common ones together. I tried it 3 times, worked for almost a year once, two weeks another time, and maybe 3...3 and a half years the third time. So it's always touch and go with that. If you want just gourami, I would do one each of...pearl, opaline, gold, moonlight, dwarf flame, dwarf honey. I would try to get them all as small as possible and get them together as soon as possible. They are very hardy, so as long as the tank is stable you should be able to do two batches of three, and not have any problems.
If you don't want just gourami you could do a nice community in the cube. How about this...1 pearl and 1 dwarf flame as the centerpieces, 6 neons, 6 cory cats, and some shrimp.