In my mind, in working on the comic, I didn't really envision them making cat noises at all, save the occasional growl or yelp that could be construed as human or as feline.
Kennedy Phillips, our sound supervisor, thought it was a travesty that the animated pilot didn't make use of cat vocabulary, though. So he inserted some as proof of concept. He and Fable eventually sold me on the idea. It just seemed to work really well with Freckle in particular, who is otherwise quite reticent.
It helps that Rika, Kennedy's cat, is a consummate voice actress. Most of those sounds were custom-recorded by holding her up to a mic.