The skin is compatible (as it is for download) with any other skin
that does not use mm_french_hats.brf and mm_french_uniforms.brf.
(I also have a mm_colours in there but that should be just some
nitpicky details It doesn't matter if that one gets overwritten by anything else.)
If you want to use it together with another reskin that uses these French *.brf files you'll have to merge them.
And for merging better look beforehand where the modified meshes in both of the brf files are.
(especially those that both use because your problem was then probably either accidentially
copying french_infantry_pants into the brf or the other mod uses french_infantry pants)
... and btw. if you clearly have modified the reskin yourself with (probably) another french reskin.
Why the hell do you ask me if I know a way to fix it?
I have no idea what skin you are trying to use it with and what resources the other reskin uses/needs etc.
PS: it does not really matter for merging which reskins brf you use as a base.
Better use the original brf as a base to put 2 different reskins together and
copy all the modded stuff from the modded brf files into the base one.
Then you'll also see more easily where the two reskins could have conflicts.
(like both of the reskins changing the same pants etc.)