Thanks for this additional info markwkidd.
After some digging, i have found these scripts that should work on indexing not recognized roms, since my problem is not only with MAME but also when creating playlists that have some SNES custom ROMS… I’ll try this at home.
This bounty you have linked seems interesting, but in my modest opinion, a more “direct” approach would be "folder X -> core X, folder Y -> core Y, scan databases for game metadata, and do not ignore those games without info, creating playlist entry with only the ROM name’. This method is used by 2 other systems and to me, they are “fine” to be used when i configure Rpi3 boxes to my friends