I hate this bit, because you clearly care about them and I hate the rough start you've had into the hobby! It's not fair, and a lot of new hobbyists do wind up leaving the hobby because of similar problems. I don't want that, I want more people joining the hobby, having success, and enjoying it.
Is there a chance of you upgrading tank size in the near future? Ideally, I'd want that for you and for the fish. You'll find it much easier to balance and maintain, and the choice of fish you can keep will increase a lot with a 20-30g. If you can't upgrade tank sizes, then I would return or rehome the cory, ideally rehome to someone with a group of the same species of cory. They can live for years, and as you've seen, a 5g can't really support more than 1-2, and condemning a second to live in a tiny tank without the proper amount of schooling buddies isn't much of an improvement on doing that to one.
Mollies can definitely get huge, given enough space and time! These old ones of mine lived for around eight years, in a 57g tank, and believe me, they used every inch of that space! Lovely fish, these got to around 4 inches and produced hundreds of offspring, so they really can get huge.
View attachment 165884
To help with scale, the mollies behind them, white and orange (gravid) female platy in front, and the blue guppy above were all full grown adults. These fish can get big given the change.
Gary was saying Prime stinks in the sense of the smell you mentioned, not meaning it's a bad product
Saying keep it simple, use a declorinator (which Prime is, Stability isn't), that Prime does smell bad, but it's all you need.