Best way to expand an ELK M1 Gold system for an ADU/Studio


I'm in the process of building a backyard ADU/Studio and wasn't quite sure what the definitive way to monitor the unit would be. I am planning on adding motion and/or a glass break sensor and a door sensor, and a secondary keypad. 
I was initially thinking of running a cat6 to the unit and connecting it to a ELK-M1XIN and then wiring everything locally to it. Would this setup work or do I have to homerun everything back to the main panel? Also could I operate that ADU as it's own zone arming/disarming it separately from the main house?
Thanks in advance for your wisdom!
Using a M1XIN wired back to the M1 in the main house using Catx cable is fine.   That's better than running multiple zones between buildings if you have more than a couple.  And if you want a keypad in the ADU/Studio, you'll need to run a cable for the data bus anyway.
The one thing to be careful of is surges caused by nearby lightning. That can damage the M1 and anything else connected to it.  To protect things, make sure to put a surge suppressor, like these Diteks, on each end of the cable where it enters each building.  The closer they can be to the ground rod for each building's electrical service, the better.  That is, run the M1 cable so that it enters each building at the same point as the electrical service.  You want any surges to get to your electrical system's earth ground over the shortest possible distance.
It would be best to run the remote M1XIN and keypad on an auxiliary 12VDC power supply.  That eliminates any issues with voltage drop over the cable.  Also gives you additional power for PIR detectors, smoke detectors, etc.
If you want the ADU/Studio to be controlled separately, you can set it up as a second Area.  The keypad in the ADU/Studio would normally be used to control that area.  But any keypads in the main house can also be used, though it takes some extra key presses to temporarily switch those keypads to the alternate area.
Thanks @Cocoonut that is super helpful and appreciate the additional info about handling surges!
Just to be sure, do I need to run an additional line back for the keypad or are you saying that I can run it to the M1XIN and run a separate 12v locally to power it? 
The keypad and M1XIN are both data bus devices, and can share the single set of wires back to the M1.  You need to be sure to terminate things properly.  Exactly how will depend on how you wire this up to the M1 with all your other devices (and any M1DBHs)