There is a difference in size, needs, and agresiveness. Mollies are very aggresive but less so when they are together. PLaties are very lovely, nice fish. One of the best livebearers. Guppies are the same way. Sword tails are less of a specitalty of mine but I know they grow to be one of the largest common livebearers. They get more agressive as they reach their full size of 5 inches. I hope this helps.