I know you really want goldfish, but fancy goldfish live between 10 and 15 years. That's a lot of responsibility for a long time. I think you will enjoy your fish more if you wait until you can house them in a larger tank. It will be less work on you which leaves you more time to enjoy them.
More water changes will help with water quality, but it can't make up for a lack of swimming room. Since there is no way to tell how large a goldfish will get, I think it's just better to wait. I love goldfish and wanted them for years before I had my tank, but I waiting and am very glad I did.