The living situtation is different for each exchange program. For mine, I got a room in the dorms, eventhough there was no rule that I had to.I figured it'd be better, easier to meet people, etc. Since my the school is small (600 people tops) the dorms here are a good mix of everything from first years to post-grads from all over. Another friend who is studying here in London has a very different situtation. She is living in a dorm that was set up for her program, so all americans. Yet another friend who is in Japan right now is living with a host family.
So lots of options for housing, but the big thing to do is to ask the programs when you're looking at where you want to go. While it's good to be around lots of different people, having an american or two hanging around is a good thing. They get your pop-culture references, they like peanut butter, and most importantly they're going though the same stuff you're going through.
Hope this helps
bf