Most FAQs I’ve seen so far on the internet about Photos for Mac have been largely based around wanting to transfer all your old photos from iPhoto into the new Photos app and then decide whether or not to have them all in iCloud. What I wanted was to start fresh with a new Photos library in iCloud but still have my old iPhotos available offline on the Mac. My iPhoto library was 85GB and this would have meant upgrading my iCloud storage plan.
Also I had some questions about how iMovie worked with iCloud Photo Library and Photos for Mac.
Hopefully others will find the below answers useful.