I think the ancient kilt Oatmealsavage is referring to is the one offered by the Celtic Croft. While it may be convenient for re-enactment or whatnot...as far as I can tell, it is not an accurate piece. The belted plaide/great kilt is however. If you poke around Matt's site, you should find that there is evidence of the feilidh beag dating back to around 1690...which means that while uncommon, I don't think that they would have been unheard of around the 1730s or so (whenever Rob Roy the movie takes place), I don't know if there is any evidence of them being seen in the '15 (around the time period of Rob Roy the novel). I could see people wearing the cut off upper portion of the belted plaide for warmth... although I think that is referred to as a plaide in later terms (nopt to confuse with plaide as in the whole thing). Although the time period when this was adopted, I have no idea.

Brings up some interesting questions. Hopefully what info I did add wasn't off the mark.