Smooth gaits have more to do with the angle of the shoulder and length of the humerus. Other aspects can contribute as well, such as the croup angle, or a roach back, or a short back. But from my reading, I've found the first two to be major factors: a horse with a short back can definitely create a smooth gait if other factors are good.
A short back does limit flexibility. Sometimes it can also contribute to forging or other types of interference, though its usually in relation to other conformation issues. However, these horses are also agile and able to carry weight well.
Anyway, looks like I'm just reiterating at this point:P