When does a Leprechaun's Ongoing ability activate?
The correct order is:


Commission is played. This triggers two abilities in order. (Step 2 and 4)
An extra minion is placed on the base. This triggers the ongoing ability of the Leprechaun. (Step 3)
The new minion has less power so it will be destroyed. But it's ability is resolved first. If any. At the end of this step, the minion is gone.
The next ability ability of Commission is now executed. And Leprechaun is moved to another base. 


The rules of Smash-up are vague and conflicting (even the rules admit this). So you need common sense to resolve this. But in this case it is clear that both actions of Commission are executed in order (Play an extra minion. Then move another player's minion from the extra minion's base to another base.)