Hi Gene,
You can turn •North America Terrain Mesh US (East & West - 2 disks) v1.1 into a coaster. It's superceded by the v2 disks. Install the Canada - Mexico v 1.2, then install the v2 Mexico.
Aside from that, install order doesn't matter. There won't be any Scenery Library entries to make or layering issues if you accept the default install path, since those folders are already present.
You might consider the North Atlantic for FSX, and the Northern Russia and Siberia for FSX to complete your whole-world coverage.
hope this helps
jt/fsg