Vacancies do not mean promotions! You don't need to have a specific rank to do a specific job, I've heard of Sgts as RSMs. (This is something that bothers me).
Cadets can skip a rank, but in my opinion it should not be necessary except in exceptional circumstances. The cadet could be given one promotion and assigned the position required (i.e. MCpl doing job of Section Sgt), most corps seem to award rank with position though.
