Yep BW2 confirmed it.
The first Gym leader of the game was the rival in the previous game, and flat out states he would have whipped the floor with you if he used his real team, (You get to fight him again in Post game and his team are in their 60s)
It's dependent on the amount of badges the challenger has. So anyone can start in an town,
Yeah, I have to say it was very humbling when I found out that gym leaders were holding back. I mean I had always suspected it, but I still made fun of gym leaders like Falkner(the first gym leader you fight in Johto) for being weak, when it turns out that they were going easy on me and could have destroyed me if they wished lol