I have only seen a select amount of 20" wheel setups that look good on a 96-04 Mustang. A few of them are posted on this website if you look hard enough.
In my opinion, 20" wheels fit the 05+ cars much better due to their larger wheel wells. On a 96-04 car, the openings aren't all that big. 18" wheels look pretty large on our cars, and I couldn't imagine wanting to go much bigger.
Will they fit? Yes.
Will you lose performance? Yes.
Will they look good? Thats all in the eye of the beholder. Put some trash donk wheels on there, and no. But again thats just an opinion.