But again, I've read that if you wear a waistcoat you shouldn't wear a belt. Am I assume that really it comes down to personal choice?
I many ways yes, it's a personal choice. I've certainly seen many people wearing a belt under a waistcoat. But the traditional view is that one either wears one or the other. If your kilt fits you properly, you shouldn't need to wear a belt with a waistcoat, and it would be hidden by the waistcoat anyway. So it would be superfluous. In fact, it would create an unsightly bulge under your waistcoat. So the question which leaps to mind is "why would one want to wear a belt with a waistcoat?"

Now the wearing of a belt over the waistcoat is another matter entirely. It, too, is occasionally done. But those more knowledgeable than I would need to explain the hows or whys of doing that.