I'm not so ready to accept that a Scotsman in 1689 would know, and use, Gaelic as a primary tongue unless he's from the Hebrides or Highlands. If he's educated, he's likely to know Lallans, Scots, a derivative of Middle Scots, better than Gaelic, and he's probably quite comfortable with French.
You are correct, you need to know some specifics before you can persume what the person's mother tongue might have been.
My hero is from the Ardgour and they spoke Gaelic as a mother tongue.
I beg to differ between the question of the mother tongue and the ability to speak a second or even a third language. The whereabouts of Highlander clans are quite clear and so are locations of the Lowlander families. Any given Lowlander family had Scots and maybe French. The Earl of Dumbarton, for instance, stayed with King James II/VII in exile in St Germain. Chances were he spoke perfectly French.
Even in the 18th century, most of the "commoners" of the Campbell (the most pro-Lowland Highland clan) spoke Gaelic as a mother tongue.
When it goes to members of the upperclass of the Highland clans we find they spoke Gaelic as a mother tongue and English and/or French as second/third language. A Campbell or McKay chief had Gaelic in late 16th, but may have not have any Gaelic at all in the late 17th century, but his tacksmen and maybe even his own family would have had Gaelic.
A Cameron, MacLean, MacDonald, MacKenzie chief (any of the branches) would have had Gaelic plus the other language(s). For instance Sir Ewen Cameron of Lochiel had Gaelic as a mother tongue and because he went to school in St Andrews he had, of course, Scots. As he turned away from the covenants -- I think -- his son John was educated in France.
A generation later, things changed drastically. MacLean of Duairt was fostered from baby days on by the Campbells and he was Presbyterian and spoke Scots... but I think he must have spoken Gaelic too, otherwise he'd not been able to personally prevent his subjects to attend the Catholic mass. But then, it says he just stood there with his cane pointing to the other church, he might have forgot his Gaelic after all. After Culloden, the chiefs and their heirs were forced out of their land and held in custody for twenty years. When they returned, they had changed and the clan system crumbled. Those 18th C chiefs spoke all Scots, and any chief born after Culloden may have had no Gaelic at all.
I mean this is what Jacobites were all about when they started out to fight. They feared for their assets and culture. And right they were.
With all that said, it is really important to be clear about what clan or family and what time your character is in, before one can decide what his mother tongue was. I envision many of the Highland Clan Chiefs as being bilingual as all of the Gaels are nowadays.
So, to come back to the original question, my hero would curse in Gaelic and back then, he'd had no Presbyterian restraint upon him either. He speaks Scots and French, too. Of oucrse he does, he's my hero after all.
murmel.