The town of Liberton where Thomas Mill was from is near Edinburgh which would make him a lowlander.

Greenock and Riccarton is also in the lowlands. Near Glasgow.

So these people may not have been members of Highland Clans.

This is the most mis-understood thing about Scotland peoples. Not everyone was a member of a Clan. In fact a very small percentage of the total population lived in the Highlands.

Today that is different but in the days your ancestors lived in Scotland it was true.