nowhere in the VGC rule is stated that you cant use volcanion http://www.pokemon.com/us/pokemon-news/the-2016-video-game-championship-format-has-been-announced/
Volcanion is dex number 721, and since legal pokemon are dex numbers 001-720 (with specific exceptions given later in the document, such as jirachi and keldeo), I'm going to go out on a limb and say it's not allowed unless they specifically update the rules to go up to 721. The page you linked was merely an announcement of the rules with a general overview and it states "The complete rules will be posted shortly on the Play! Pokémon rules and regulations page," implying there's more to the rules than just what's on the announcement page. In fact, if you go here and click on "Play! Pokemon VG Rules and Formats" it will download the official complete pdf file of the rules for you to read as much as you'd like.