"Wallace" = "Welsh" ('foreigner' in Saxon parlance), ie indigenous Romano-British, which included south-west Scotland, unlike the Anglo-Saxons who settled up the east coast, or the Gaels who came from Northern Ireland and settled western Scotland further north beyond Strathclyde. "Highlanders" seem to be almost entirely Gaels. I guess by Wallace's time the Lowlanders were pretty mixed, but still notably more Welsh in the West and Saxon in the east, and never very Gaelic.