I would probably live with Twilight... if that wasn't obvious. I think that she would be a great room mate though I would be open to living with AJ or RD. I kind of like the idea of having a castle to myself though.
I would go with Nightmare moon. I love her voice, her design, and the high goals she has. I want to be Nightmare Moons cute little pet... Umm excuse me I should probably stop there.
I choose my OC's race based on what I imagined my self being IF I was a pony. I also like the concept of magic, the unlimited amount of things that can be done with it, and the power that comes with it. So a bit of booth.