Went to college and stayed in a dorm from 1986 to 1988 - I'll try to dig through the cobwebs to let you know. Answers apply to Kansas State University.
1. Long twin was the only choice.
2. I lived in an all male dorm. The rule was females could visit at any time - they just couldn't spend the night. Definition of spending the night? She had a tooth brush. Only one womens bathroom in the dorm was on the first floor.
My girlfriend (future wife) lived in an all female dorm. Men were allowed in the dorm until 10:00 on "school nights" (Sun - Thur) and till 12:00 otherwise (Fri - Sat). Men could go to any room - with an escort. Before you could enter the hallway, your escort had to yell "Man on floor" so the other women would know there was a guy around. She also had to yell that when you came out of the room. Guess it was to keep women who were going from their rooms to the bathroom in only a towel from getting surprised. There was only one bathroom for the men, on the first floor.
There where coed dorms, but generally, it was still segregated by either floor, or wing. No cohabitation.
3. Cost? Oh - boy. Stretching the memory cells now. Like Paul, it was tacked into the overall bill. $200/month? Maybe? 50% guess on that.
