Don't add any more fish until the ones you have are healthy and well. Something obviously isn't quite right at the mo, be it your water, your tank maintenance, the quality of the fish you're buying, too many meds... It's something. Keep doing these large water changes regularly as clean water is the best cure for many things! I'm thinking especially for your neons who look a bit unhappy.
The platy will be fine alone until your tank is healthy enough to add another fish.