Once we bought 4 Panda Cories and one of them had torn fins. I really did not want to treat the tank with medications because I knew we had one prego Platy and I didn't know if it would hurt the mommy fish or not. The little Panda's fins healed pretty quick and were completely fine within a couple of weeks. If your fish are getting along, and nobody is being picked on, just keep doing your water changes and the fresh water and clean tank will help him heal. I know you are watching him like a hawk so if something changes in his behavior, etc. you may have to get some medication. Also, keep a check on your cories bar bells at their mouth because sometimes they can get irritated. Cories need the sand or rock substrate in the bottom of the tank to be clean because they are always looking around for food down there. It is really best to have sand or soft rocks on the bottom that are smooth, instead of the sharp rough kind of rocks. But, if you have the sharp rough rocks on the bottom, just keep them clean and try to get something else when you can. Are the cories sharing the shrimp pellets? If they are that may be enough because usually cories will hunt around and find flakes and other food that the other fish miss. If the cories seem to be not wanting to share or kind of fighting over the shrimp pellets you can add 1 or 2 more. If you notice any food laying around on the bottom of the tank, then you are feeding them too much so cut back just a bit. Cories are great little tank cleaner uppers and they are so cute!
Also, with your Platies, being prego is very normal for them and there is really nothing you need to do special but if they are going to have babies you will need a breeder box for when they have their babies to keep them from being eaten. A breeder box that will keep mommy fish on the top part and let the baby fry go into the bottom section is best because mommy and other fish in the tank will eat the babies. Please update this if anything changes and let us know how they are doing!