Is there a way to transfer just the settings and rates of a certain set of photos from one catalog to another one?
Lr native (e.g. catalog import, or xmp reading..) does not allow for picking and choosing which items to transfer, so you'll need a plugin, or some such cleverness.
What I would do:
* Import all (subject) photos using "Copy" to a unique (root) folder, and allow duplicates. (or you could import them into same folder as original, but be sure to use a special filenaming token for recognition - either way works).
Then, run RelativeAntics. Select the newly imported "duplicates", and make sure their target brethren are properly seen as "relatives". You'll need to edit advanced settings to customize the develop settings and/or metadata to be transferred as part of the "antics".
Afterward, delete the imported dups that you no longer need.
There are no doubt other ways to do it, but that way will work, if you do it right, and the plugin works as advertised... - let me know (outside the forum) if you need help with any of my plugins.
Back everything up *before* you embark.