As noted, the majority of Scots are lowlanders, and Anglo-Saxon, while the inhabitants of the Highlands and Islands were Gaels. Of course there were some weird exceptions, like Galloway being Gaelic until early part of the game period, and Inverness and Caithness being Lowland Scots.
Since it's "culture," not "language," I'd assumed that the idea was that the Scots, like the Bretons (Brythonic and Gallo-Romance) and Silesians (West Slavic and High German), are treated as a culture which transcends linguistic features. Not that I agree with that—the Scots have always distinguished Highlanders and Lowlanders, and I'm not aware of any such distinction in Brittany—but there's still a case to be made for it. Well, the main case is that if Scotland survives as an independent country, it will lose Gaelic as an accepted culture the moment it starts colonizing, and if England eats it as per usual, it will ethnically cleanse the Gaelic areas as quickly as it does with Wales and Ireland.
In a mod I'm working on I added Cornish, Manx, and Highlander as cultures. (Among others. And I split Brythonic: Bretons, Welsh, and Cornish from split off from Gaelic: Irish, Manx, and Highlander, because until people started romanticizing the Celts in the nineteenth century, the two groups had nothing in common aside from a propensity to get conquered while making heroic last stands.) My mod will likely never see the light of day (I started adding provinces and nations wholesale without testing) but the point is that adding cultures and changing the cultures of provinces has to be one of the easiest changes to make, if it bothers you. You edit the culture file, alter the province files, and add Gaelic as an accepted culture in Scotland's file (primary culture if you feel like it) and you're pretty much done.
