Supposedly the federation is a lot more diverse than humans with weird ears, noses or foreheads. Just that the old shows did not really have the budget and / or ability to make convincing, actually alien looking species. TAS can get around this by being a cartoon, modern shows use plenty of animation.

But even in TNG is a running joke about the cetacean officers that are always off screen of course. But supposedly there is a whole part of the ship under water where intelligent dolphins are doing their duties as crew members.

Kzinti are Star Trek canon.

Niven himself wrote The Animated Series (TAS) episode ‘The Slaver Weapon’, adapted from his short story ‘The Soft Weapon.’

Niven sees Star Trek as a separate universe or timeline, but it shares the Kzin with his own Known Space universe.

In Lower Decks, there is an ensign who is Kzin in addition to Dr T’Ana being Caitian.

Also, it was Niven himself, writing on the official Star Trek website, who put forward the view that Caitians and Kzinti are cousins, with the Caitians having settled on their planet Cait, and adopting a more scientific and technologically oriented culture.

In TAS, Lt. M’Ress was the Caitian second communications officer.

Caitians are cousins of the Kzinti, like Vulcans and Romulans.