So this might be a very stupid question, but I understand that for categories such as Any%, Dark Side, and Darker Side, u can buy only one moon at the shop. R u allowed to buy anything else at the shop while running that specific category, or can u only buy just one moon?

You can buy anything at a shop but it would be a waste of time most likely so people don't usually buy much.

Earlier Darker Side routes included 3 outfit moons for an additional achievement from Toadette, however, 500 faster moons have been found and so the current routes doesn't buy anything but moons from the shops anymore