Secondly, if we look at what Highlanders actually did in combat there is little reason to count them as light troops or skirmishers. Its true they were very mobile and that they could move quickly but when it came to fighting they formed up and required good ground to successfully deliver their charge. Duffy deals with this in detail and it's well worth reading.
I'd rate Highlanders in the British Army as regulars. I believe that the style of swordsmanship taught in the Highland regiments was not that of the clans. Certainly I cannot recall any Highland regulars giving a volley and dropping their muskets to charge. They seem to have done as British regulars did. So well motivated regulars seems to fit best.