Aggression might depend on the type of platies. I doubt it would be a serious difference, but you might not have to worry about aggression with different types of platies. however, platies, in an over all view are in no way aggressive.
The most aggression Ive seen between males is a little show off skirmish where the males would flare, circle eachother and then engage in a biting brawl. Platies dont have the physical features to seriously cause damage to any fish. The only damaging result COULD be a male being singled out and shying away from normal behavior, resulting in decreasing health.
My 30 gallon has between 5 and 10 platies (havent counted them) and all are males. They do fine.
My 20 gallon has about 20 platies, all female and one male. They range in size and age. jsyk
a good ratio is 2:1 or 3:1 female to male. breeding is inevitable.