bettas have a habit of attacking brightly coloured fish, platys are brightly coloured fish
there are some success stories, but unless you know your betta's character inside out and know he's chilled out with other fish, it's not recommended
I used to put platy fry in with my betta fry when they got to 3 weeks old to encourage them to eat non-live foods. They were the same size and got on well till around 6 weeks old when they would just lay into them. Must be all the hormones raging round
So I personally do not mix my bettas with anything other than cories, plecs and apple snails