Do fish really need to be in groups?

I have a 55lr tank and 6 lovely little fish. A bristlenose pleco, Rosie barb, 2 dwarf gourami (M&F) a spotted molly and a Halloween molly. I was thinking about getting a couple more of each molly as I have been told they should be in groups but mine seem completely contented and happy with their other tank mates. Could I just get a few more completely different fish instead rather than a load of the same?