Opinions are going to differ on this one but ill tell you what factors make me say that many goldfish in a 75g tank is going to be an uphill battle with the odds stack against you. I know people are going to probably disagree with me. So, Here is why I say that it would be a bad idea. I know the footprint math versus number of fish would say other wise but in this case I wouldn't use that due to the Bio-load volume involved. It alone is going to put so much stress on the system and it's just going to increase as the fish grow. The maintenance most likely required for the tank is going to be not only very costly for one, with the volume of water required for the weekly maybe twice weekly changes you may need to keep everything in check. Then, factor in the time consumed on maintenance which I believe would slowly increase overtime due to fish growth.....I'm not saying it cant be done I just don't see it being sustainable in the long run.
Now, I'm only sharing the obstacles I see from my personal experience keeping fish. It's your tank! Your the one that decides whats an acceptable cost for your hobby.
Hope it helps