Spawn rules for the Overlord
Since you weren't sure which version you played, I'll post my answers for both editions, although i'm significantly more familiar with 1E rules than 2E.  

From the First Edition Descent Rules:
The overlords turn consists of:

Step 1: Collect Threat and Draw Cards
Step 2: Spawn Monsters
Step 3: Activate Monsters 

You spawn monsters before you activate them, where activating them is considered moving and attacking.  Again, this is for 1st edition, and I haven't played second edition so I'm not familiar with the 2E rules and the PDF doesn't want to load for me right now.

The Second Edition Descent Rules seem to imply that you place and activate the monsters at the same time, in their respective groups(p. 8 &amp; 16).  The FAQ doesn't have a specific clarification on the issue.  

I would honestly be surprised if the overlord wasn't able to place and activate monsters in the same turn.  In 1E, you only expect monsters to last a round or two when thrown into direct combat with the players so I would expect the same from 2E.