The fishes pose a threat to nauplii due to the vast size difference. If you want your cherry shrimp population to thrive, you'll need to take steps to protect the nauplii. Very young cherry shrimp < 1/4 inch will be seen as a snack by platies, especially if there aren't enough hiding places for the shrimplets.
Adding more plants, especially dense and bushy varieties, or shrimp-specific hiding caves will significantly improve the nauplii's chances of survival. Moss can also be a good option as it provides a safe haven for the tiny shrimp.
Nauplii need biofilm and fresh algae to survive the first 2-3 weeks, after that they will start to go for larger food.
Botanical like cholla wood and catappa leaves are good support to grow biofilm.