Regarding that one there is a lot of politics in the actual series which is relevant to the answer. It isn't *just* that having 3 S+/SS class mages in the same unit is ridiculous. There was someone fairly powerful who isn't really a fan of Section 6 for other reasons, and they were able to convince enough other people based on that to put extra limiters on them beyond what's normal for mages of that level.
I haven't watched the series since it aired, but if I ever rewatch it I'll go back and add an answer expanding on that. I don't actually remember exactly where all the information is. Nan…