How well-known are the Kzinti, really? I imagine seeing them in Star Trek would boost sales and interest in Larry Niven's novels.
Oh no doubt they would. IIRC Larry Niven was all for the use of the Kzinti in season 5, and of course he'd been quite happy to adapt one of his own short stories for the animated show. So there's clearly no issue on that
As to how well known they are... Ringworld
is a Hugo and Nebula winning novel and was quite a hit in its day and is still often cited as a classic. There's nowhere near as well known as Wookiees, obviously, or most other media aliens... but as far as aliens in sci-fi literature, they're probably among the more recognizable races.
Farscape had some fantastic 7-foot-tall kitty people, using actors on stilts, with excellent costumes and makeup. I can't really see the forehead-obsessed Trek going for something so different, but I'd have loved to see them try.
I'm not sure who you mean offhand. Unless you mean the guy from "DNA Mad Scientist", and he was pretty awkward looking. I love Farscape, I love their muppets, and I love their commitment to doing really weird looking aliens, but the truth is their muppets-of-the-week - or partial muppets of the week - were of decidedly variable quality, and he wasn't that convincing.
the Cowardly Lion, not too bad for the day.
'For the day' is probably the key phrase.
The Cat Nurses in Doctor were pretty awesome looking, IMHO
No idea what that is. Sounds like one hell of a B movie, though. The Cat Nurses in Doctor! See the thrilling feline medical assistants to host themselves in the body of the medical professional! A tale beyond your imagination!
And I suppose the probably-Caitians of The Voyage Home
looked alright, but we never had to see them talk.