Ok, there are virtually no rules about having people in your rooms or whatever - technically if you have a visitor from outside your college, you're meant to sign them in and they're only meant to stay a certain number of nights, but within college there's no problem. We all live on mixed staircases (in my college, at any rate!) anyway. Everyone's an adult, so why wouldn't it be allowed? However, Freshers' week antics on a shagging scale were somewhat quiet compared with what I've heard at other unis, so don't get your hopes up too much. I didn't pull and neither did an awful lot of other people.
As for the interview, I've no idea what sort of stuff they'll ask you, but basically they'll test you until you can't do any more.