It'd probably just be easier to have Garlean soldier mobs cast a longer spell called Alarum or something that would spawn additional soldiers. Maybe a threat level system in Garlean outposts where killing mobs in them places a debuff on you that increases aggro radius on other mobs.

As for the mobs in this expansion, the logic for them I think is not aggressive, but rather territorial. We come in on their turf; there is no "neutral ground", rather us being the lone invader. The only real difference with the Garleans is their level of organization.