Can you delete a migrated Photos Library without losing any photos?

Hi - In principle, yes - if you are sure everything you need is is now in the photos library.

However, you won't get as much space back as you think, because that library is now sharing the original image files with the photos library via file system links. It reports its size as though it also has those files - even though it doesn't. So when you delete it you will only get back the space taken by the database, and some support files such as thumbnails or previews - which is a relatively small amount of space compared with the original files.

Also, bear in mind there are some file formats (especially video formats) that are no longer compatible with later versions of MacOS. If your old libraries had any of these, then they won't have transferred. It is a good idea to archive a copy of the migrated libraries to an external drive, in case you want to go back to them at a later date to find any such files.