How to Decouple iCloud-Sourced Photos from the iCloud in Photos Application
You cannot in Photos designate some photos as "local" and others as "iCloud Photo Library". It is all or nothing.

However, you can have locally stored photos outside of the Photos application, which will be untouched by iCloud Photo Library.

Regarding your comment about storage capacities: In Photos Preferences under the iCloud tab, you can set your storage policy to either (a) store photos and videos in full resolutions on this Mac, or (b) store photos and videos in full resolution on iCloud primarily and only cache photos/videos in full resolution locally as available storage allows.