After you select a Movie Theme, you'll be taken to a panel in which you can select which effects are applied and what intensity the effects are applied at.
Unfortunately, beyond that you can't do a lot of customizing. That's the challenge of an automatic feature.
You can break apart instant movie to view all the effects and transitions applied to the clip. But it is not reversible. i.e, we cant move back to instant movie once it's broken. Undo would work of course
The screenshot you mentioned is called PIP or picture in picture . You just have to scale every footage on different layers.