I won't presume to answer your questions, as there are so many here better informed than I. However, having once made my living as a writer, I found it useful to keep a few reference books on hand, especially as I wrote somewhat technical nonfiction that could easily be shot full of holes if I wasn't precise. I suggest you buy a copy of Early Highland Dress by Matthew A.C. Newsome. It can be had from The Scottish Tartan Museum, but they can be frustrating to deal with. Only recently did they respond to an email I sent them over six months ago.

Further, The Scottish Dirk, by James D. Forman will answer your questions about the sgain-dubh. It's easily had from Amazon. Neither book is very expensive.

There is also relevant information here: http://www.tartansauthority.com/highland-dress/

You didn't mention a time frame for your story, but depending on how far back you go, you must take into consideration that much of what is discussed here as being traditional highland dress, is in fact of recent origin, and would be strange to the eye of a 17th Century Scot, for example.