If its what i think you are talking about I _BELIEVE_ those gear things you are referring to are ABS sensors. There is a sensor that uses those gears to tell how fast each wheel is spinning, so that when there is a large difference it applies the brakes.

If changing all 4 sets of pads and need too turn rotors due to surging. turn all four rotors or just front two?

I'd do all 4 just so you have a baseline for next time.

I just finished all on my 07, replaced the rotors/pads - should be good for a while...