20 GB is not that much. Actually neither is 1TB when working with pictures regularly. I am wonderung how I can archive my material, when the cloud is full. Until now I have stored lrcats for each year locally. Will this be the way with the new lightroom as well?
Just like any cloud service if it is full, it is full. You will need either more space or to archive on an external drive or other cloud service.
No you cannot do this with Lightroom CC. The thing to realize is that Lightroom CC is a completely new program. It is not the update to the Lightroom we know and love. It does not work with lrcat files. It can only have a single catalog that has to be completely stored in the cloud. It cannot export this catalog to a local copy. If you anticipate going over 1 TB all the time or need much offline storage, this is not the program for you and you need Lightroom Classic, which is the update to Lightroom proper as we know it. It allows local storage and can selectively sync only certain images (albeit only as smart preview) to the cloud contrary to Lightroom CC.