Ok, I have some of those goldfish too. The thing is you need a bigger tank. Sure you can wait until they get bigger. That's if they live that long. They can't heal properly without the required space. They may heal and they may not in that 10g tank. For these goldfish they need about 15 gallons for the first one and 10 gallons for everyone after that. If you don't want them for too long then the 10 gallon may only provide them with a year or two of life and you can move on. Pet stores don't tell you about this because most times they don't know or don't care since they are basically feeder fish and they figure they won't live long anyway. That's why they have so many diseases and even the non feeder goldfish kind of get put aside for care to the tropical fish. If your ex's father has all these big expensive fish then maybe you should see if he has a larger tank that he can loan or give or sell you.
The temperature you have it at is close to the top of what goldfish like. Goldfish are cold water fish (compared to "tropical" fish) and nn my experience goldfish like between 62-74 degrees F. Mine is right around 70-72 so it should be fine. But don't add other tropical fish because it can be stressful for one or the other depending on the temps of the tank.
Just because the fish were clean looking in the store out of a bunch with 25% dead doesn't mean they are disease free. Can you walk the street and tell who is sick and who isn't? Same with fish. If they were in that tank they may be clean and maybe not. So just be aware of that and that they probably came with whatever is wrong with them. But getting them out of that situation is the first step for survival.
I know you are trying to take care of them the best you can. And I give props on that. That's how I started with the ones I have, well my fiance wanted to save them from their fate.
As for the issues with them, as stated they are some of the most hardy fish out there. One of mine when it was young had a large hole in it. It was about an inch and a half, maybe two inches. It had a whole the diameter of a common sharpie marker. The LFS (Local Fish Store, not Petco or Petsmart) told me just marine salt. So I added a bit of marine salt, forgot how much exactly. And sure enough in about 3 weeks time you couldn't even tell the fish had a hole in it. Now that same fish is almost 7 inches and my largest goldfish currently. It's been about 15 months since the hole. The fins will heal on their own. Things that might cause them to get hurt like split fins could be bad quality fake plants, sharp rocks, aquarium ornaments that are too small (mine swim anything they can wiggle through), etc.
Anyway good luck with your fish. I hope it goes well.
I might sound like a curmudgeon soon.
