Details of cuff, collar and coat length may pass in a small scale like 15mm, but the give-away will be the color of your troops' small-clothes, their breeches and waistcoats, their coat turn-backs and their cross-belts.
In the 1740s and '50s, through 1764, small-clothes were supposed to be colored wool, blue for Royal regiments, red for all others. Coat turn-backs were to be the same facing color as cuffs and lapels.
In the Seven Years War, starting in the 1750s, there was an unofficial change to lighter-weight canvas (osnabrig) small-clothes, in white or drab tan, at first in hot climates like North America and the Caribbean, later spreading to the regiments in Europe.
The 1764 uniform regulation changed the shape of coat collars and cuffs, and made official the change to buff small-clothes for regiments with buff facings and white small-clothes for all other regiments. The 1764 regulations also specified that coat turn-backs were to be in the color of the small-clothes, rather than the facing color. The 1764 regulations were in force for the AWI, although they were much modified in the field.
The color of the cross-belts also changed. In the 1740s, belts were buff leather by regulation. During the SYW, some regiments started whitening their belts. In the 1764 regulations, white belts were made standard.
So, your WAS red-coats should have red or blue breeches, facing color turn-backs and buff cross-belts, while your AWI red-coats should have white or buff breeches and turn-backs, and white belts. I can see the difference, even in 6mm scale.
There is a sweet spot, in the middle of the SYW when the uniform was unofficially changing, when you can pick a regiment with white or buff facings (and therefore white or buff turn-backs), paint the small-clothes likewise white or buff and their belts white, and they won't look too out-of-place for the AWI.
You're welcome.