The problem is that if you remove those synced photos from LrClassic, that will result in all of them also being deleted from the cloud when you next enable sync in that catalog. Which is probably not what you want!
Yes, you could leave sync paused before removing the images from the Classic catalog, and the cloud images should then be safe....but it's a bit of a ticking timebomb which will go off should you inadvertently resume sync in that catalog. So probably the safest thing to do, assuming that the LrClassic catalog is only for "learning purposes" at this stage, would be to delete the catalog completely, then start another new catalog (give it a different name), but don't enable sync in the new catalog until you are ready to do so (now that you understand what happens when you do that).