Most kilted weddings on Scotland tend to be over rather than under dressed: in black Argyll jackets and waistcoats with silver buttons and black brogues, or black PCs and black brogues. However, a few weddings opt for day wear: tweed jackets and waistcoats with brown or black day sporrans and brogues. If my memory serves me well, I think Princess Ann's second wedding adopted this dress code. Occasionally I have seen people in Jacobite shirts and potains (is that how you spell it?). This is about as informal as a Scottish kilted wedding gets. I know of only one wedding that really went for kilts and boots. Friends of my son (in their late 20s or early 30s) who are climbers were married on a Scottish hillside. Whatever you wear, have a great day.