I'd personally deworm them and treat them with hex shield. If you've had one getting like the previous one, you'll have more and you'll be pulling your hairs out. They'll just go one by one. It's no fear with platies because they also multiply but the fish are more stressed when they carry parasites. It actually can take months for the disease to properly show up. Look out for white stringy poop too. It will not be visible in each platy. As a person that's kept platies for quite a few good years, I'd consider yours on the small side, they can in fact grow to that size in 3-4 months.
This is not to say that they're damaged. I'd just consider treating them once for worms and protozoa.