Okay, so I sort of figured it out for people interested. If you have ANYTHING from a mod that sets the game event to christmas, the christmas pokemon will show up. They aren't spawnable using any NPC spawners so you have to wait for them to appear NATURALLY (though increasing spawn rates does make them appear easier). Notably, the christmas versions do not have modified pokeball sprites (which I feel the author may want to do, primarily to ensure there's no visual confusion between using a normal charmander and a christmas one). I haven't seen any halloween variants of any pokemon yet so I don't know if the author has actually made any or not.