As Mark said - only one library in each user account on each device can sync with iCloud, and this library has to be the System Photos Library and iCloud Photos will merge the libraries from all devices into one central iCloud Photos Library.
To put it slightly differently: iCloud Photos is primarily a syncing service, keeping the System Photos Libraries on all your devices in sync and identical.
I like to think of iCloud it like this:
- I am using the same Photos library on all devices, and this library is stored in iCloud.
- Each of my devices is holding a local shadow copy of the one iCloud Photos Library and this library is automatically syncing with iCloud.
- Whenever I work with my photos on one of the devices the changes will be updated in iCloud and from there synced to the other devices.
- It looks like we were modifying the local library, but actually our device is just a handle to modify the central iCloud Photos Library. This is the main reason why iCloud Photos does not suffice as a backup, only as an offsite storage. A backup needs to be a separate safety copy that we do not touch. It cannot be the dat we are continually working with, as that will not protect us from user errors.
As you are using iCloud Photos on several devices with different system versions and different storage capabilities, consider what you want to store in iCloud:
- Your device with the least storage will be the bottleneck for the maximal size of your iCloud Photos Library. The iCloud Photos Library needs to fit on all your devices, at least with the "Optimize Storage Option" enabled. But "Optimize Storage" will be no joy, if you want to work with your photos also, if you are having no internet connection or the network is slow or very expensive. And a fully optimized library will be noticeably slow, even with a fast internet connected.
- Not all codecs for videos or images are compatible with all system versions. Some codecs we could use in iPhoto or Aperture will be causing problems on devices with macOS 10.15 Catalina or later or iOS/iPadOS 13 or later when trying to upload to iCloud. Incompatible items can cause the iCloud syncing to hang and should be removed.
- I would not turn on the "Shared iCloud Photos Library" as long as some of your devices are using older system versions,
- At least one of your Macs should be using iCloud Photos without using the "Optimize Mac Storage" option, so you can easily backup complete versions of the library by keeping clones of the library or running Time Machine.
BTW, I really enjoy iCloud Photos. It is exactly what I have been longing for, while using Aperture, limited to My Photo Stream to keep my libraries identical across my Macs. It is just too bad that Apple did not offer this service together with a professional successor to Aperture. Having to fall back on third-party photo editing extensions is breaking the lossless workflow, as we no longer revert individual adjustments.
