Can you use pokemon like regirock or dialga in pokemon official tournaments

There are 3 Kalos (Generation VI) legendary Pokémon, and 4 Kanto (Generation I) legendary Pokémon available in Pokemon X & Y.

Kalos Legendaries

• #716 Xerneas / #717 Yveltal
  • Found as part of the story (Team Flare's HQ) at level 50.
  • Version Specific (X has Xerneas, Y has Yvetal)
  • Both are stationary
• #718 Zygarde
  • Available after the Elite 4, in both X & Y
  • Found on the bottom floor of Terminus Cave at level 70
  • stationary
• #719 Diancie
  • Available only through a special event
• #720 Hoopa
  • Available only through a special event

Kanto Legendaries

• #144 Articuno / #145 Zapdos / #146 Moltres
  • Randomly encountered wandering after defeating the Elite 4.
  • You can only encounter one, which is dependent on which starter Pokemon you chose:
    1. Chespin -> Articuno
    2. Fennekin -> Zapdos
    3. Froakie -> Moltres
  • They will flee automatically 12 times when you encounter them, after which they will go to the Sea Spirit's Den where you can finally battle them. Source.
• #150 Mewtwo
  • Available after the Elite 4, in the cave in Pokemon Village:

    After defeating the Elite Four, a man will stop blocking a cave within the Pokémon Village. In this cave, you will find nothing but a solitary Mewtwo with its back against the door. Interact with it and a cutscene will occur where it notices you and attacks. After defeating it, you will receive the Mewtwonite X or Y item depending on your version. - Source

Not for Charizard, at the Stone Emporium in Lumiose City you can buy for 1,000,000 before and (as low as) 10,000 two other Mega Stones after you beat the Elite Four at the end of the story.

I bought Blastonite and Venusaurite. I can't only buy other Chairzardite Y or X it depends from your Pokemon version.