Please note that I am no Flygon expert. Do not take my advice as the undisputable truth.
1. Flygon usually plays Broken Time-Space as their Stadium of choice, simply because it accelerates the deck a lot. Of course, I have also seen variants that do not play any Stadiums just because of the mirror matchup.
2. That is a hard question, as they are two entirely different decks. Pink Flygon is focusing on board control, while Dark Flygon is focusing on speed. It depends on your play-style.
3. I would not combine them, personally. It would be inconsistent.