Its been a lot of years since I had platies but my memory is along the lines of what Dumashi is saying, I just don't ever remember platies being a problem for aggression and to me it always seemed one of the easiest fish to stock in any number that suited my circumstance, exactly the opposite of so many types we talk about a lot here where particular grouping numbers really matter (I'm talking all male platies when I say this, as I agree if you have females they should outnumber the males to divide up the harassment.)
~~waterdrop~~