You might also find some interesting information in the records of the American War of Independence. Thereios quite a lot of data and the Americans publkish quite a boit on this. The Scots are over represented in NON scottish units.
Laird, Guid stuff! The 94th Scotch Brigade were never termed Highlanders (though they were erroneously ordered to omit Highland from their title in 1809). The old Scotch Brigade were formed in 1568 and although on the Scots establishment served with the Dutch army. To their chagrin when they returned to the British fold their ancient lineage was disregarded and they were numbered 94 (they considered themselves senior to the Royal Scots)! Sadly they were amalgamated with the 88th Foot in 1888 and became 2nd Bn The Connaught Rangers though retained 'Blue Bonnets' as their march past.
That is truism about the old British Army history, but during the 1770s the scots were particularly prevelent, according to a book by Dr Edward Curtis " The British Army in the American Revolution". ISBN 0 85409 906 9 He puts forward evidence that recruiting was slow in England ahnd Ireland, because the war in America wasn't popular in England and there were a series of good harvests in Ireland. Combine this with the Highland clearances and good bounties there is a steady flow of Scots recruits from lowlands and Highlands. So in 1778 2/3 of the 15,000 new troops raised by indiviuduals and towns are from Scotland.
You might also find this an interesting source for your other subject of study, as the same author discusses the "Loyal Volunteer Regiments" by Manchester, Liverpool and Edinburgh for the American War. These are units paid for by public subscription with some officers nominated by the towns. This is middle class involvement in the army, a development which rather worried the King.