RTF is a global modification that replaces all towns in the game.

Wow, that's my main idea – if the Nagas were taken from Heroes 6, why can't the others be? The demons look great there, as do the undead and orcs.

Because no one involved in the mod development can get the models from Heroes 6. The Naga mod was created by another person (if I'm not mistaken, a Pole).