In reply to Jamie.

That's the exact course of action I'm in the process of taking with my Agency's 300+ sites.

"Classic Editor" offers dire warnings not to use it on a site that doesn't have Gutenberg, but the first thing the plugin does is check to see if Gutenberg is installed, before making any site changes, so there is absolutely no risk to a non-gutenberg install.

As for "Classic Editor Addon" it interfaces directly with hooks provided by "Classic Editor", so will make no changes to any non-gutenberg install.

In short, you are perfectly describing the intended use case.