shared media library, stuff not working since moving to MU

Hello,

I was wondering if I could have a shared media library for all sites running in the Multisite network? I only found this article so far: https://blog.channeleaton.com/sharing-media-libraries-across-network-sites

Otherwise, would it be easier to set up a specific FTP folder where all media are uploaded manually and then to use the URL field to insert media in posts?

Also, I recently set up the multisite network, everything works fine so far except for 2 things:
-WP does not take into account the custom sizes I have set up for pictures in the Settings > Media panel. (=my main problem)
-Jquery tabs no longer work correctly. The tab’s content disappears when clicking on the corresponding tab. Other Jquery elements work just fine and use the same js/css files… And it still works on the monosite install.