I have currently synced my Lightroom Classic CC catalog to the cloud based Lightroom CC, and so all photos are showing up. I would like to add photos from Lightroom 5 (a rather large, and important catalog) however a message pops up that I can only sync one catalog at a time. Does this mean that the photos currently synced will be removed from Lightroom CC?
What you could do is upgrade that Lightroom 5 catalog to be compatible with Lightroom Classic CC. Then merge that catalog into the catalog you are synchronizing with Lightroom CC. Then all of those photos that are in the Lightroom 5 catalog will become part of the catalog that is being synchronized with Lightroom CC and can then be included.
How do I go about upgrading that Lightroom 5 catalog to be compatible?
Just try to open it with Lightroom Classic CC. When you do, you will be prompted to allow Lightroom to upgrade it. After the upgrade is complete, Lightroom will open that new upgraded catalog. Now, it will be necessary to switch back to your other Lightroom Classic CC catalog that you have been using and then import from the catalog that you just finished upgrading.