Reply to Connecting X1 to External Mixer

I just bought the Traktor X1 and have it setup to my S4 for now at home so I can practice with it. I have been taking my S4 to gigs that had no gear , but recently I started doing gigs that either had the Rane 57 or the Pioneer DJM900 mixers. So now that I have the X1, I want to bring only that and my Mac to connect to these mixers.

So a few questions...
1) Do I just connect my X1 to my laptop and then my laptop to the USB on these mixers?
2) If so, is there a need for me to connect to the Aux In (on the Rane 57) or any of the 4 Lines on the Pioneer 900?
3) Do I need to download any drivers etc?
4) Usually I am the starting off DJ, so what needs to happen when the next DJ after me comes on?

Thanks!