When does a summoned creature take its first action?

Can a summoned creature get a full round's worth of action on the round in which it is summoned, or does it only act on subsequent rounds?

Does it always act on the initiative of the caster who summoned it, or does the creature roll initiative independently?

1. It acts on the next round.

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.

Thanks, Alex!

Is that spelled out anywhere in the rules?  I spent a long time searching both the core book and the PC for any guidance on the matter.

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?

No, it's not spelled out anywhere in the rules. It probably should be. 

I derived it from the notion that a character acts on his initiative number, and a newly-summoned monster doesn't have an initiative number.

This is rather timely, since I've been working on devising a formula for creating ACKS stats for Pokemon.

Unbalanced Pokemon are some of the most fun:

Shuckle - AC 12, Mv 10', HD 1+1, Attack: Constrict 1d2

Fragile as anything if you can hit it, but...good luck hitting it.

I've always felt Magic Missile needed a soft buff. 

"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"

Someday I'm going to perform surgery with magic missile. Just use Detect Poison to locate the tumor and then don't miss. 

Magic missile does no damage to inanimate objects, and can’t target specific parts of a creature. Hard to do anything fancy with it.

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.

This calls for custom spell research!

One might interpret a tumor as an independent creature... 

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