yeah.... sizes mentioned are illegal...
but so is breathing loudly in vic
but cars come factory with wider rims on the back so of course it's not an issue there
just buy the rims you want and be done with it... you are putting them on a skyline, so you'll get defected eventually no matter what rims you run so you might as well have the car looking how you want
also those sizes will look like aids
need to look at 18x9+20 all round as a MINIMUM