Previously, the Merton Street houses have been only guys, and there have been a few guys in Rose Lane, all on the same floor. Most of Rose Lane, however, is girls. There are no mixed floors.
At least, that's how it's been so far. As I said, your year is the first they're mixing properly, so I'm not entirely sure, but floors will probably get mixed as well.
The only restriction I've heard is that they're not going to put girls on the ground floor of Merton Street, for their safety.