Syncing photo libraries with the new Photos (Yosemite) on three Macs

Question

Dear list members,
I have three Macs running Yosemite 10.10.3 and a very large photo library (160 gb). I have locally copied this library on all three Macs and manage them with iPhoto, trying to keep them identical to each other as possible with Photo Stream. Yesterday I upgraded to the new Photos app and converted the photo library on one of those Macs (I made a backup of this library in iPhoto format on an external HDD, just in case).
My goal is to have all three Macs with identical photo libraries (locally copied), perfectly synced and available also on my iPhone (for download). So my questions are:
1. How will the new Photos app and the iCloud Photos service deal with those three libraries (which are basically the same, except for a few differences) on the different Macs? Will all of them be synced and become identical after a while, even considering their large size?
2. Just to be sure that the photo libraries will be identical on all three Macs, should I delete the libraries on the other two Macs, let Photos build new ones, and let them be synced through the new service iCloud Photos (to which I subscribed with enough virtual disk space)?
Thank you very much for the attention.

Answer 1

If you are going to use the iCloud Photo Library, I would create new, blank Libraries for the other Macs, set as default.
Then, when you enable Photos and iCloud Photo Library on those Macs, it won't sync anything from them. You can then re-link the old iPhoto Libraries back to iPhoto on each Mac and manually add any missing photos that were only on those two Macs. Just Export the photos from iPhoto and upload them to the iCloud Photo Library.
Ooops, almost forgot.
To create a new Library or to Link to another Library, hold down the Option Key when you start iPhoto.