If I understand things correctly - Highlanders of the period might well were a shirt/chemise type garment (laced or tied) with a wrapped and belted plaid over that. so - no buttons would be necessary - just a belt...

Lowland Scots would be wearing basically the same thing as English of the same period.

Note: for some reason many people have problems referring to a wrapped and belted plaid as a "kilt". therefore I didn't.