Another good reason NOT to mix western and highland apparel is that each evolved for completely different circumstances. It's been discussed many times on this board that kilts aren't really all that good for riding. Highlanders wealthy enough for a horse would have wore trousers while riding.

Cowboy boots were developed specifically for riding. The pointed toe allows the rider to quickly get his foot into the stirrup while the heel keeps the foot from going completely through the stirrup. This keeps the rider from being drug along the ground because of a trapped foot.