No animals in the lineup, please. She wouldn't fit in with the rest in the merchandising.

Except Tiana was an animal for a time.

Yes, Tiana was an animal for a time in her movie, but not her whole movie.
Nala is an unofficial Disney Princess. There's 11 (or 13) offcial DPs;
Snow White
Cinderella
Aurora
Ariel
Belle
Jasmine
Pocahontas
Mulan
Tiana
Rapunzel
Merida

and maybe
Anna
Elsa

Not officially, but she is technically a "Disney Princess". Just like some of the official Disney Princesses aren't even Princesses.

SentinelPrime - but Tiana has a human form they can market.

Kind of, in a tongue-in-cheek way. I'll accept she's not in that line-up they created for merchandise purposes, but hey, she was betrothed to a prince and ends up queen so I guess that's gotta count for something!

Also, is Mulan ever royalty in any way? It's been a long time since I last saw that movie and I'm gonna watch it again soon, but I don't remember her ever being or becoming a princess.

^She's not royalty, but sh fits in with the others: "I want" song - check, romantic interest - check, animal friends - check, villain - check, good looks - check, fancy outfit - check.

Actually, Nala is a queen. And if we follow this animal rule, then Kiara and Bambi's mom and Faline would have to be DPs.

Then half of these Disney Princesses don't even deserve to be Disney Princesses then. They all get married and become queens or just wives.

1. But they're not queens at any point in their movies.
2. Well, actually, some of them might never become queens; they might rule a princedom.

She's technically a queen she married Simba at the end of the movie