I'd like to have specific photos stored in my iPhone and specific ones stored only on iCloud, so I thought to have two different Photos libraries, one "optimised" and one not. Is it possible? And how can I manage them from my iPhone? Or is it a way to do something like this?
It is not possible to have multiple libraries with the stock Photos app on iOS.
However, you could use the stock Photos app to hold all the photos that you want stored on iCloud, and a seperate, third-party photo app to hold the rest. The third-party app could be something like Google Photos, Flickr or Amazon Photos.
Hold down the option key on your Mac's keyboard, launch Photos. Now you can create ore select a different library. And in this article also is explained how manage the iCloudPhotosLibrary https://www.lifewire.com/use-photos-for-os-x-multiple-photo-libraries-2260028