Yes, there are hidden slots for the 3 event Pokemons after the nº 718 in Pokedex, just like in Black2 and White2 with the 3 event Pokemons.
Also the new megas from ORAS can be traded to XY.
Since Diancie is a gen VI Pokémon, of course it will be added to your Pokedex!
If you see here, Diancie is in the Central Kalos Pokédex.
Hope this helps.