Would a cap from an older Mustang fit, because I've seen some aftermarket billet ones that I would like to put on my Mav.
Yes, they will fit. Like Allen said above, they may or may not be vented but they will twist on the Maverick neck just fine.
Not all Maverick caps will twist on just fine. Some caps have more "ears" than others. Some caps, once installed, stick out farther than others or are too close to the body. Some would be so close to the body that they will not go on at all. bottom line....it is almost a guessing game when it comes to gas caps, but the later year cars used the caps with 4 "ears", while the early cars used the caps with only 2 "ears". If you find a cap you like....but it won't fit....don't give up. All you need to do is find the correct gas tank "neck" and it will fit on your car.