Greg has never attacked a single Spellcaster in my game though the only ones I play as are my own (so no check on this with pre made ROM sims). I have maybe half a dozen of them.
He didn't attack any human sim with the incredibly friendly trait with max charisma even prior to introduction. These two also had max mischief in common. Not sure if any of that factors in.
Otherwise, unless they're a child or pregnant, he's attacked everyone else. He especially hates my lone Apex female weresim as if she were Vlad. She's my only Apex so far. I can't keep a green bar with Greg. The conversation *always* goes to unpleasant between them no matter what and they end up in a brawl. So I gave up on Greg knowing my Apex female.
He does genuinely seem to enjoy Spellcasters the most of the occult types in my game.