I had a tank full of aggressive young male platys and just a few females (they were brothers and sisters) - which caused a lot of stress and some deaths. I tried to calm things down (and to reduce the incest somewhat

) by introducing some big girl platys - but the boys weren't at all interested in the newcomers. They still got pregnant though (so you never know what's happening behind the bushes!

)
In the end I only calmed things down by separating the boys from the girls entirely. I think you'll soon notice if any of them get stressed (and it's just as likely to be the boys) - they lose interest in picking at the algae and clamp their fins. I wouldn't worry unless that happens