Yes, mesh made for 9 will work in X.
Mesh that was made for 9 will only have a single LOD (level of detail) in each file, whereas mesh files for X can contain many LODs. Depending upon the area, X has better resolution (by a long shot!) than what was in default mesh for 9. So adding 38m FS9 mesh for the U.S. won't do much good, as FSX has 38m default coverage.
There are some freeware mesh products for FSX, that are just as good or even sometimes better than what some payware offers.
If 9 mesh is used, try to have as many LODs as possible. If you have 38m you would also want 76m, 152, 304m or what's commonly known as "buffer mesh". The display in FSX for the highest available LOD is not that great, so any improvement from say a single 38m file won't do much good.