They were fighting among themselves enough as it was. The only people they liked less than other clans, were foreigners. Their own chiefs used them against each other, q.v. James VI's writs of Fyre and Sword.
Frankly, if you lived within range of Highlander deprevations, I doubt you'd apply the term quisling to the Black Watch. Maybe to some of their masters, but not the actual soldiers, as their loyalt was to the head of the clan.
Policing here was more a case of protecting the rest of the country rather than conquering the Highlanders.
Roads eventualy did that far more effectively.

P.S.
In 1667 the Earl of Atholl was commissioned by the king to raise companies to keep "watch on the braes." This of course was to protect the Lowlands. Who protected the countryside from them is an enitrely different matter. Charles II was their king, so would you still use the term quisling?

BTW, how much of Gaelic being banned is true and how much is a myth like the banning of bagpipes?