In “Quentin Durward” there is no doubt that they are the Scottish Guards. Maybe later they changed to Swiss. I just don’t know but Scott invented the historical novel and always had the history accurate.
I took a quick look but haven’t found any such Scots unit in the service of France at that time; the late 1700s. However the Swiss Guards were well known. See: http://military.wikia.com/wiki/Swiss_Guard. In the War of the Spanish Succession (c. 1704) there was a small unit, apparently, of Scots in the French Gardes du Corps (cavalry). The British raised Scots regiments after the creation of the United Kingdom. Maybe the author you cited was talking about a UK regiment and not a French one?
I found the “Quentin Durward” problem. That novel was set in the early to mid 1400s. Swiss guards came in later. They’re famous for guarding the Pope, and in France, as the king’s royal guard. At that time they wore red coats faced white. There’s a monument in Switzerland to their heroic last stand.