In P5 or 6, you can save any individual material you want, and apply it to any material on any other figure/prop.  The Material Collections that P6 will save can include all or any of the materials of the figure/prop.  You can select any you want to save from the list.  They, however, will expect to see the same materials from which you saved them.  Will give you an error, and ask if you want the materials created for you.  Probably not a good idea ;)

Both of these options are great for props, which normally won't take a MAT pose.

INJ poses usually just inject morphs, though you can make a hybrid file that also changes the mats.  Often, separate files are in the same folder.  Depends on which is easier for your set.

I'm doing a set of props right now in P6, and making separate MT5s for P5.  I will have to have someone check them in P5 for me, and also the geometry.  You can't really claim compatibility unless you know for sure.  BTW, I have WordPad as the default app for opening any of those files.  If nothing else, you have to change the version number.

John