If I've migrated from iPhoto to Photos, can I safely delete the migrated photo library?
If you have been using Photos only since migration you can safely delete it. The purpose of the migratedPhotoLibrary is so you still can use iPhoto with your old pictures (and perhaps import new ones to iPhoto only, if you have done this, then no, don't delete it.) After deleting it you can delete your iPhoto app as well.

https://support.apple.com/en-gb/HT204655