Congratulations on the Model X! Haven't seen anyone answer you so I'll give it a shot.
I'm from the San Francisco Bay Area. Here, all of the toll transponders are run by Fastrak; they can be used for bridges, express lanes, and some parking garages. I know of internal transponders (designed to mount on the inside of the windshield) with and without switches, and external transponders designed to mount on your license plate. The switches indicate how many occupants are in the vehicle and are used for certain express lanes; here, EVs can set the switch to "3" (the highest number of occupants) and receive free or favorable pricing. I think most of what I wrote is applicable to you in Southern California as well. I'm not sure how interroperable transponders are, but it's my understanding that a transponder issued in the Bay Area can be used on a number of toll / express roads in Southern California.
I drive a Model S manufactured about a year ago. Some versions of the Model S (including mine) have a metallic coating on the windshield that prevent the internal transponders from working; some of us have taken to putting our transponders on the rear windows. I'm not sure how an internal tolltag would work on that gorgeous windshield you have on your X. Have you tried looking on the Model X forums (try the Interior/Exterior subforum)? Or asking your DS? You might need to do some experimentation (if your order your tolltag see if they can send you some extra velcro strips so you can experiment with placement).
Hope that helps!
Bruce.
