I've read that dwarf gourami's can be kept in a 10 gallon, but really it should only be one.
All my gourami's are in the same 20 gallon, had I done enough research BEFORE i stocked my tank (i did tons afterwards) I wouldnt have done this, but since they all get a long great, I don't feel any need to return any of them, there was a little aggression by one of the platinums, but I put more plants in the tank, and now all the gourami's get along and coexist with no problems at all, they look healthier than they did at the store.