2. It rolls its own initiative. That said, if neither the caster nor the summoned creature has an initiative modifier and it's a large combat I often will have them go at the same time just for ease of play.
INTERESTING. This is one of those little deviations from normal that I glossed over because ACKS was so much like D&D. In 3.5, you cast summon barbarian, and then the barbarian immediately punches someone, almost like a convuluted fireball.
In ACKS, the barbarians are vulnerable for a moment, and hypothetically could all be cleaved through. Unless the wizard went last and they win init next round, but if the wizard went last, why didn't you shoot him?
"On the far end of a deep chasm is the mechanism to release the treasure, a small actuator that must be hit. a pile of rocks sits on your side of the chasm. On all sides of the actuator to release the treasure are actuators that will destroy it instead, such that a missed attack throw will destroy the treasure."
"I cast magic missle, I hit automatically"
It is not summoned berserkers, but conjured elementals that demanded the ruling here. I had a PC who'd taken to throwing charging air elementals like artillery shells from 360 yards away. It became disruptive.
And yeah, the rules from 3.x are the ones that caused the alternative default interpretation.
fun fact: if you are an alchemist in pathfinder, you can have a pet tumor: http://www.d20pfsrd.com/classes/base-classes/alchemist/discoveries/paizo---alchemist-discoveries/tumor-familiar-ex