I don't think you've upset things by adding more fish. In a 20 gallon tank, this is not overstocked by any means, and stocking slowly is a good idea when you are starting up a tank. Panda cories (I've kept these before) are happy in a group, yes, the bigger the better, but I think a group of four are fine.
Hmmm. Secluding himself. The first time reading your thread, I thought parasite. A lack of appetite, lethargy and hiding is sign of this. But so is just stress. You mentioned he had a nick out of his dorsal fin, but you don't have any nippy fish. Did you get your panda cories from the same tank, same LFS? Was the panda tankmates with nippy fish in the fish store? Was the nick there when you got him?
Those questions are a bit all over the place, I was just trying to find out if there's a chance an internal parasite (or an external one you can't see) could possibly have been introduced with your new fish 2 weeks ago, or if something's stressing him out. If it was your tank parameters or conditions, all your fish would be exhibiting symptoms, so it's not that. But I don't like to add meds in unnecessarily.
I will let you know that I had a fish acting this way recently. I decided in my tank, along with the way some other fish looked or acted in the tank that it was internal parasites plaguing the tank. The good thing about dewormers and even some antibiotics, they don't necessarily upset the biofilter, if done properly (not overdosed and meds not mixed together unless compatible). And I've researched a lot lately. So I ended up dosing the tank once with PraziPro (praziquantel). The fish acting like your panda cory was pretty much back to herself the very next day. She is eating enthusiastically, regained her top strata, not hiding anymore. Liquid Praziquantel is great for many different parasites (but not all) and gets absorbed by the fish, so it's good when fish aren't eating, as opposed to medicating food, but pricey.
How's your platy doing now, is he/she eating, or still not too hungry?