Parallax effect in the set

I'm not sure you want the layers to be all semi-transparent when creating this effect, that will just be cluttered.
Instead try masking off parts of the front layers to reveal what's behind them.

Since Motion is a 3D environment you can make it do the parallax effect for you.
Organize our layers the way you need them, then add a camera. Now push the background layer back along the z-axis and scale it to taste. Next move the second-from-the-back layer back, but not as far back as the background and scale it. Repeat with the rest of the layers.
Now when you move the camera from side to side the Parallax should just be there.
For added realism you can turn on the camera's depth of field.

Have fun