Fingers in mouth is severely underrated. Though putting fingers inside a cat mouth isn't really a great idea.
You just have to be careful. At the side is kinda iffy although not that bad 'cause typical cats don't have a ton of jaw pressure, and of course right under the main fangs is bad. But right in between the two fangs, right at the front, cats have just a really tiny flat ridge that's barely teeth. I guess so as not to bite their tongues? Anyhow, you can stick a finger in there and if they bite down nothing happens at all.
Whether that's very similar with cat girls I couldn't say, unfortunately.