Dunno of anything highland specific for the 16th C but there are plenty of Border Reiver ranges for the lowlands. Perry do some Irish gallowglass and hand gunners suitable for the 16th C which might be the source of some conversion pieces.
The earliest, specifically highlander, 28mm range that I can think of are the Eureka Highlanders in their ECW lineup so 17th C not 16th. No two handed swords but bows, lochaber axes etc.