File Sharing Mac-PC results in Permission Changed and Locked Files
Thx douggro.

I've just finished solve my problem and you helped a lot. Os X Server was an easy solution. 20 minutes between download and success.

On Yosemite client I already created a groupe yesterday, that I basically called "Sharing".
With the file-sharing Options of OsX Server I had more options to set this groupe as I intended to do, and more particularly by spreading permissions to children files and folders.

OsXServer also allow you to create dedicated folder for groups which is a quite good solutions also. But didn't even needed it since the files-sharing options did really good !

So now my wife can access the server with dedicated rights, download stuff and PLEX catch newly arrived files, on the go !

Thx !