You can include the IMDb ID in your folder names → https://support.plex.tv/articles/naming-and-organizing-your-movie-media-files/#toc-0
Alternatively use the IMDb ID as the search term in the fix match dialog (or TMDb/TVDb IDs… however those will require a tmdb- or tvdb- prefix respectively) → see “Tips” in https://support.plex.tv/articles/201018497-fix-match-match/
So… if your request is to allow mixed content libraries, I recommend you comment/vote in the existing feature suggestion on this matter in order to avoid distracting/dispersing votes and attention.
