I would say Tympole, Plapitoad, Seismitoad, Golett, Golurk, and Eevee are all in the game due to the statue from the preview, though that also isn't guaranteed as we had Pikachu Statues in Nimbasa.
Given the reveal of Sylveon, I would say it's practically guaranteed that at least a number of Pokémon from previous generations are going to show up, some of which may even have new stages of evolution. So while we're still waiting for confirmation on the Pokémon they've shown, I'm kind of leaning more towards them being included rather than being used as substitutes.
The thing I hated most about Gyarados' typing is that in gen 1 it was originally going to be a water/dragon. But since this typing makes it only weak to dragon, and the only offensive dragon move was dragon rage and twister (I think), GameFreak decided to change it's typing so it wasn't overpowered.
But in the later gens they added more powerful dragon moves (such as dragon pulse an outrage), but GameFreak still left Gyarados as a water/flying. WHY!!!!!!!!
With all honesty, I would even prefer Gyarados to be a mono - water type, if of course it doesn't become a water/dragon in gen 6.