[@Scribe of Thoth] Aeromancy is unsanctioned for two main reasons: it's the magical discipline used by a particular tribe of unruly savages that descend from the northern mountains every so often (the tribe themselves call their magic Hrideki), and for reasons still unknown Photepi Sorcerers can't seem to get it to work the way they can. 

Pyromancy is a sure bet. As for the Sekhmet aspirations, remember that the Red Orders choose who they want, not the other way around. Maybe your character has family in the Sekhmet, or is generally positive/ambivalent in attitude toward them. Remember also that most Sorcerers are distrustful at best of the Red Orders. Sorcerers that join their number are treated with pity, as most think turning their gifts towards warfare cheapens them. Sorcerers that try to maintain their status in their Cult alongside their membership in the Red Orders are treated much more contemptuously, and are generally black sheep in the Cults.