Multiple libraries in Foto

Giovanni,

when you migrate an iPhoto Library to Photos, it will create a new library for Photos, but both the old iPhoto Library and the ne w Photos Library are sharing all image files using hard links. Even if they are showing as both the same size, the disk space is only used once. You can delete the iPhoto Library, but will not gain much free storage by deleting the iPhoto library, so keep it, until you are sure that the migration worked well and you do not need the iPhoto Library to retrieve books or metadata that did not transfer. See these documents for background reading:

• Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support
• Six Colors: The (hard) link between Photos and iPhoto

At least keep a backup of the iPhoto Library and the iPhoto application. Photos does not transfer all metadata tags from iPhoto to Photos.

if there is a way to also import two other libraries I have in iPhoto so that I can get rid of iPhoto once for all (I use another editor to post process and as such Foto is just a quick way to keep the old collection of pictures not included in my main archive and, especially, those shot with the mobile phone).

Photos has no tools to merge libraries. You can only export the photos from one library and import them into the other library.