Aberchirder was known as Foggieloan prior to 1823 when it was renamed and according to the information I can find "Foggie" was "created " in 1764.

That aside I've recently read a book on a school in the outer Hebrides, where in the Hebridies by 1900 the general practice was children wore kilts but adults wore trousers. This may explain some aspects of the picture.
Also to a certain extent the protestant churches were controlled by Edinburgh headquarters / majority population which frowned upon the kilt.

The area you are talking about is part of the crossover between the highlands and east coast communities, the coastal communities are much more connected with the outside world, intermingling with not just lowland families but those further away due to the fishing industry. But I wouldn't call the area as part of the Scottish lowlands.

Even as early as the 1700s the influence of southerners "fashionable" dress in trousers was spreading north and west before the clothing restrictions.

The Scottish lowlands are generally thought as below the Forth and Clyde, with a cross over area maybe including Fife and Stirlingshire. There isn't a border marked saying highlands this way, lowlands that ( except in advertising blurb).

While the area is more farming than crofting, it is part of the highlands, Gaelic was spoken much more there then, than now. As to what your ancestors wore, you'll never know. It could be some form of kilt or trousers or if wealthy ( or skillful enough with the needle) either when it suited.