Nothing good can come of a co-ed sleepover at this age, or at any age for that matter. They can all see each other during the daytime/evening, when parents are awake and supervising. I am surprised that any parents would be willing to host such a party.
IN a recent conversation with my boss (a pretty high-level executive at a well-regarded company based outside of DC), he said that he and his wife allowed his 17-year-old daughter host a co-ed party with 4 couples after Homecoming. My boss and his wife went to bed at about 10:30 pm, and they set their alarm for 2:00 am. Then at that time, they went downstairs to make sure all of the boys had been picked up by their parents. Then only the girls slept over, and the host parents went to sleep.
It seemed like 2:00 am was kind of a late time for the boys to leave. I think I would have said 11:45 pm or so. I dont want to ruin my kids' social lives, but my parents (who were pretty lenient) would not have even contemplated allowing boys over until 2:00 am (back in the early 1990s).