When I play Showstopper, can I move more than 1 unit for the showdown?
No, you cannot move more than one unit with Showstopper.
According to the RiftJudge FAQ (ID 4591), Showstopper allows you to move only one unit. Additionally, as noted in the related FAQ (ID 4987), when you move a unit with a spell like Showstopper, that unit must attack alone; you cannot move other units alongside it.