Cool, crazy idea.
I guess the main decision is working out what a standard soldier should look like, and quite how much better the musketeers should be. In Twilight the best Fubarnii warriors would cap out at about 3 stones (more for cavalry). I expect that the musketeers would be more like 4 or 5 stones, with plenty of stamina abilities so they can hold off a number of bad guys.
I assume the musketeers would all be able to activate each other?
I'll look forward to hearing what you come up with! Be sure to share it all here