Don't quote me as I'm not certain, but back in my FordForums days, I remember everyone saying that they could run XX size and diameter wheels on their base model Falcons etc due to the XR's in that series having XX size wheels.
I think it goes off the largest available factory option for that series. So for example a BF Typhoon came with 18's, but optional 19's therefore you could get away with 21's if the rest of the dimensions were within the guidelines...
As far as translating that to Skylines, it's only really the 34 that has anything 2" bigger from GT-R to GT-T, with 33 GTR's only being 1" bigger than GTS-T's, but that'd still allow 33's to run 19's as long as the widths and tyre sizes were within the guidelines...
Think there's also another rule of any wheel/tire upgrade having to be within 15mm of the largest/smallest factory option wheel/tire package for that car...(included smallest as you can't run 13's on a car that should have 17's etc)