Syncing a 'catalog' has no effect on your local (Lr Classic CC) catalog.
Stay with the cloud system or not, your local Lr and catalog is untouched.
Put the files you want synced into their own collection(s) and only sync them.
Once you enable syncing there will be a little double arrow looking icon to the left of collections, click it to enable/disable that collection for syncing.
All other files/images in your catalog will remain on your local system.
You could have 2TB of images on your local drive(s) yet decide to sync only 1 of them if you so choose.
Maybe a test with just a 1/2 dozen images to start. Learn the system with those then decide.
Thank you for the response. I wouldn't say that I need to "learn" the system, other than how Adobe handles the cloud storage, which is woefully inadequate (20 GB) at the photography plan price point. While their storage option may be convenient and necessary for those that are technically challenged, there are certainly other cloud storage options available that are cheaper and most likely more secure. I really have come to appreciate the features of LR 6 but from what I have been able to ascertain on this site it appears that it is doomed to a slow death of no updates in favor of the subscription model that Adobe is now pushing. Time to cut bait and find an alternative I guess. Once again, thank you for responding.