I have read that the a square set up is ideal for a mustang, is this true, why? Any issues with putting 20" wheels on a GT PP, such as factory foundry wheels and/or any recommendation for 20" wheels, thanks.
Ideal is dependent on what you want the car to be used for. And no issue with 20" wheels as long as they clear the PP brakes.
Project 6GR wheels come in 20's, and can be offered square(I believe, I know 19's can), and are lighter than the stock PP wheels.
There are quite a few people running 19x10 and 20x10 square setups, and quite a few different companies make wheels in those sizes. Check the top of the wheel/tire sub-forum for a thread that's stickied about wheels.