Restore photos from old iPhoto library backup to new Mac Photos app
I had the same problem. You've got to do:


Open PHOTOS while pressing OPTION key. Choose form a list your old
iPhoto library. Either Photos will convert the old file or will
direct you to a link on Apple where you can download an utility that
will convert the file for you.
In any case you will have to repeat and open Photos with option key pressed. Choose the converted file.
Once it is opened you have to select all photos and EXPORT them to a folder in your desktop.
Then you go through your filtering. After having all the photos you want in a folder, reopen Photos with the option key pressed. But now choose your regular Photo Library.
Then you IMPORT the photos from the folder.
In case you need the convertion utility from Apple, here is the link:
https://support.apple.com/pt-br/HT202358 . (It did not work for me. I think El capitan doesn't like these old utilities - from 2013.)


Hope you succeed.