Ok, first off, I don't personally believe any peppered cories belong in a 10g tank. I certainly do not suggest adding another. They actually get pretty decently sized when they are adults. Females reach 3 inches and are kind of chubby. The males tend to stay smaller. They are not hugely fast growers, so you are probably ok right now. Also, cories are not tight schoolers, so they are not a fish that is going to stress out because you have 2 instead of 3. Personally, I have not seen much activity from a lone cory, but 2 or more does better. Ideally, it would be good to add another, but you are seriously pushing the bioload of a small tank as it is, and I do not suggest adding more to that.
Mollies get bugger than people realize. They do not stay as 2 inch fish. They do better in a larger tank. I suggest 20g or bigger for them. Platies stay smaller, and they do fine in 10g tanks.
Do you happen to know about cycling a tank? Here is a link in case you do not.
http://www.aquariumadvice.com/forums/f12/fish-in-cycling-step-over-into-the-dark-side-176446.html
Some people cycle a tank without fish, but you already have fish. No worries though, everything can still be done, and we can help you get a nice stock going. Keep in mind, a 10g tank is pretty small. You will not be able to cram a bunch of fish in there. All the fish you see at pet stores are usually babies. They will grow. There are some amazing nano fish out there that really thrive in 10g tanks, but they are not as commonly available. Are you able to re-home fish if necessary? If you are, we can talk stock. If you can't my best suggestion is to keep your water as clean as possible and do your best with what you have. I hope you can get a bigger tank over the summer for the mollies though.
I realize some of this has been said, I am just shoving it all into one post for the ease of reading.