The "rules" for wearing highland attire are pretty much like the rules for any other sort of attire. You wear what you wear because you want to fit into a group.

So if you want to wear Highland attire, you should find out what the "rules" are, and then decide how you want to follow -- and/or break -- them.

Certainly one of the rules used to be that kilts are wool and kilts are tartan, but today at any Highland games in the USA you'll see plenty of people wearing kilts that are neither. Opinions will vary on that, and this thread really isn't the place for that discussion -- the point is, those people are doing the same thing as the others -- they're wearing a certain garment a certain way to show they are part of a particular group.

It's all very tribal.

Contra-intuitively, one also signifies being part of a particular group by not following the rules of a group (if you dress unlike those in the group then one is part of the group called "outsider"). Frank Lloyd Wright was a great example of this. He deliberately set himself apart as not being part of the group (the group called "normal") by designing clothing for himself that was unique.