Do you have to play through any previous versions of the DS Pokemon to fill up the Pokedex in XY? Not that I mind, just wondering if my XY Dex will be missing any if I don't transfer them.
To fill up the Pokedex, you will have to use the Pokebank feature, coming out in December, to transfer Pokemon from older game. Not every Pokemon is obtainable in X and Y.
So yes, you will have to play through a couple older versions.