I think the only requirement is that FSDT KLAS will stay above the US City scenery in the Scenery Library, which is what will happen by default, if you install FSDT KLAS last.
This scenery order causes a big hole behind the E-Terminal and in front of runway 25L/25R. I had this too. I fixed it by changing the order that FSDT is lower than US CITY X LAS VEGAS. Additional you must rename the KLAS-BGL in US CITIY X scenery to .off. And important, you must set the KLAS-BGL in standard scenery to .off too!
Additional:
First I had an issue that my IFR departures were canceled to VFR flights at least after takeoff from runway 01R because of overlaying ATC-borders and I couldn't switch frequency and send my answer as fast as required.
Please check an IFR departure from runway 01R towards north-east.