Short answer - yes. I use UTX USA and UTX Europe with Orbx Global
textures and they work very well together, just like you might have had
UTX and GEX textures working together. I did have just that, but
uninstalled GEX and installed Orbx Global instead. Using Global
textures, there is no need to worry about scenery priorities.
But... if you have an Orbx scenery area, like PNW or Central Rocky
Mountains etc, etc, it's a bit more complicated. These do the same job
with roads, landclass and 3D objects within their areas as UTX does
(only even better, I reckon). So to prevent conflicts and weird
problems in areas where they overlap, you need to ensure that the Orbx
scenery area(s) have a higher priority than the UTX files in the Scenery
Library list. That means listing them in the Scenery library above the
UTX entries (i.e. with a lower number).
If you install the Orbx scenery area after the UTX area, that will
probably work out right. Or if not, you can move them up or down the
list manually. Also there is a tool in the Orbx "Central" configurator
which can be used to lock the Orbx entries in the list above some
particular scenery entry, like the topmost UTX entry, so the priority
will always stay right.
Orbx have their own "Vector" scenery area add-ons coming out shortly,
which will do the same job as UTX over large areas, but until then I'm
happily using UTX USA and UTX Europe with a whole raft of ORBX scenery
areas.
Mesh...Don't worry too much about different mesh scenery files. It
doesn't matter much where they are in the scenery priorities. FSX (and
presumably P3D) will use the one with the best (highest) resolution. (A
rare case of common sense breaking out!).
Ariel