Princess Koopa was a tag that started popping up on images of princess_peach alternate_species koopa, but that seems like a very specific scenario to have its own tag. Should we apply this to the other princesses (daisy and rosalina), or alias it away to another tag such as koopa or alternate_species?

Alias to princess_peach, add alternate_species and koopa manually.

Yes, please alias it away to something. That naming system is flawed because it opens up too many more names for just one character, and it makes a mess when names delve into TWYK and suddenly that "Princess Koopa" was actually Bowser that someone crossgendered.