You're actually not adding a new pokemon species itself, you're adding a new form of an existing pokemon. It's like adding rift gyarados.. As such, gyarados has 2 different forms itself, its mega counterpart and rift counterpart. With this you can add diff variant pokemons (ex: delta pkmns from insurgence), hackmons (too op tho) , you can even add fakemons but it won't be registered in the pokedex as a new species