I read that in certain regions, Poison Pokémon aren’t allowed out of their PokéBalls. I was considering going on vacation to Kalos with my Skuntank. Would it be okay for him to leave his PokéBall and walk with me if he doesn’t use any poison-based attacks?

Certainly! Kalos has no such restrictions on tamed poison-type pokémon, and even then, there’s really no need to worry that your skuntank will do any harm to the environment. As an animal-like pokémon (as opposed to the literal embodiment of pollution, as in the case of the trubbish and grimer lines), skuntank do not passively leak toxins into their surroundings. They simply release toxins when threatened, so as long as your skuntank is calm at all times, the two of you should be fine.