You asked about the wearing of breeches instead of the plaid by other highlanders than the 42nd. In the notes accompanying the plate of Frasees Highlanders (78th foot) in the book Military Uniforms, of America - the Bra of the American Revolution there is a reference to this. In the winter of 1759-60, an officer of the regiment wrote "...the philabeg is not at all calculated for this terrible climate. Colonel Fraser is doing all in his power to provide trousers."

Whether he succeeded or not is not mentioned. According to Pere Chaubard in Military Uniforms in Canada 1665-1970 a similar proposal in the winter of 1757-58 met with such an uproar that the regiment retained highland dress.

Incidentally, if the plaid was unsuitable for cold winters was it not also unsuitable for tropical summers? Did the 42nd wear the plaid in the West Indies in 1758 and 175P. I have only one reference to a wounded officer whose highland dress was much appreciated by the French colonial ladies. Highland dress wasn't worn in India until the nineteenth century I believe.