Iphoto and 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.

No, a migrated Photos library actually shares hard drive space with the originals from iPhoto. You only get back the space for the previews, faces and the database itself.

You certainly can put a Photos Library on an external drive, but I am not sure how that adds security. It won't by default be backed up by Time machine.

Yes. by security I thought you meant someone looking at your images without authorization. Extra backups are good practice. I have a time machine backup, a clone backup and a third backup of my library on an external drive. The only fault is that they are all in the house so a thief or fire could get them all. Yer_Man, on this board, keeps a set of backups in his car for that reason. If my work was a little less security conscious (It is like a Roach Motel - Media goes in, but can never get out) I would keep one there.

not as much as you night think, at least as long as the Photos Library is still there. Photos and iPhoto use a thing called a hard link to actually share the hard drive space for the originals, so that space is not recovered.

Just copy the library. Albums are just lists of images, what really counts is the number of images.

Photos should not be running, though just copying the library should be OK.