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IMDB.com has a list of the voice actors for the game.

The main characters you mentioned are voiced by Gianpaolo Venuta(Jason Brody) and Michael Mando(Vaas).

If you clear the outposts, most of the enemies you face will come from missions - when you start a mission, enemies spawn at the mission location no matter how many outposts you've cleared.

Random encounters will also occur even after outposts have been cleared - for example, you may stumble onto an ongoing battle between Rakyat and Pirate forces while moving through the wilderness.

That said, most enemy forces will be eliminated if you clear all of the outposts in the game. If you prefer Rook Island to be a peaceful place, then you may want to clear these outposts. I prefer to leave the outposts alone, because I find that exploration gets boring when there's no enemies other than animals.