Congratulations on your new fish.
Platys IME are a joy to feed because they are greedy pigs!
Mine eat flakes happily, bloodworms and daphnia with enthusiasm, but go absolutely wild over peas. Just ordinary frozen peas, slightly boiled deshelled and crushed. They also eat spinach and chard and I have given them broccoli. No need to invest in a huge piranha tank: I get to see all the feeding frenzies I want, just dropping in a couple of peas.
They're omnivores, so a nicely balanced diet might include live food (or frozen or jellied bloodworms etc) once a week and maybe vegetables twice a week.
