How does adding new characters throw off the original continuity? It's not like they can't simply add characters to something like their eventual wedding without being confusing. Unless a character or their actions specifically contradict the movie's continuity, then I don't see the problem, especially when they flat out stated that Rapunzel and Eugene do still end up being married at the end. Plus, why bother making a TV series out of a movie if you aren't going to make new characters? It would make the world seem very small if we only stuck with the characters from the movie.