Garran: Some NPCs break the rules about who can learn/know what, but his choice of magic school is perfectly reasonable - like the rest of the order in town, he's a monk, and monks work with Psionic magic.
Thing is, if you look at the spellbooks he sells, they tend to be mainly either Priest or Divine Magic spells (or both). No Psionic Fire or Mind Stab sold at all. Also, his dialogue suggests that he offers healing, and Psionic healing is rather limited (only Heal Wounds, Cure Lesser Condition (why doesn't he sell that spell? It would fit with the other spells he sells), and Sane Mind).
Then again, Lord Braffit should have sold Holy Water spellbooks (but doesn't), and Anna should have sold Mind Stab.
Of note, Lord Braffit is one of the two creatures that can drop the Holy Water spellbook when killed (the other is a single special drone encountered in Arnika), so it would make sense for him to sell that spell, especially since he also sells Banish.
By the way, I actually saw one of his spells fizzle during the fight. (Then again, I saw my new Bishop's Paralyze spell backfire, but it didn't seem to do anything; not even an "UNAFFECTED" message.)