Offers will not usually come out until about the same time your UAC offer for ANU comes out. Annoying, I know, but halls don't want to say yes to someone who doesn't get in to ANU because doing so is practically a logistical nightmare. Occasionally, early offers do get given out though. I think early offers are usually given from Johns and maybe still Burgmann though. It's really unlikely that you'd have a roommate. I know someone from Bruce who shared with a roommate and from my understanding it was completely voluntary (the upside is that you don't pay very much in the way of rent).
Yes, it's fine to play an acoustic guitar in your room. You usually have to stop by 11pm or 10pm during exam time, or if your neighbour knocks on your door and tells you that they're trying to sleep/study. Additionally, every residential place has a music room or space away from your room which you can use, so if you do want to play guitar at midnight (or any other time without someone hearing you) then you can just use a music room.