Q: Photos vs iPhoto libraries

1. Yes, but if you decide to use iPhoto for some reason, you won’t be able to import the Photos library into it. Note that they both reference the same files on the drive, so deleting the iPhoto library won’t free up anything near 106GB of space.

2. See above.

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Yes but it is not a good idea and will not save much space as the Photos library uses hard links to the iPhoto library for the masters and previews - Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support - and if you want or need it in the future it can be difficult to get back

LN

Is there any issues doing this?

At least keep a backup of the iPhoto Library on an external drive.

We have seen several cases, where suddenly a problem turned up in the Photos library that only could be fixed by repairing the original iPhoto Librar i iPhoto and repeating the migration., for example, if the connection between the edited versions and the original image files had been out for some photos.  Such problems can go undetected for a long time, and you will only notice them, when you try to edit or export the photos.   So keep the original iPhoto Library until you are reasonably sure, that all photos migrated well.

you are welcome

LN