Photos will not start removing or 'optimising' space until you start to run out of hard disk storage.
The best way to achieve what you want is to create a new library and then use that as the primary library. Photos will then create thumbnails for every photo and only download the original if you edit the photo or wish to export it - the size of this library will then only be a couple of GB.
However, as someone mentioned, you should make sure you have an additional copy of your photos stored safely. If you only have a single version of the original stored in iCloud then you do not have a backup.