I don't know if I have all the answers, but I will give my two cents on some of these.
It had slipped my mind that it wouldn't be your summer, but of course it's not - you live in Australia! Mid-year fic exchange sounds fine to me - I mean, it's not inaccurate.
As far as knowing who we are writing for and who wrote a story - for Secret Santa we only knew who we were writing for, but not who was writing our prompts. But this isn't Secret Santa so we don't necessarily have to do that rule. If we don't reveal them at the time of posting, I think we can reveal the authors at the end. I'll let the others weigh in on this as well.
I'd say around 1500 is a good maximum if we want to keep it to shorter stories, though there were some in Secret Santa that were longer than that - but if we want to keep these stories shorter than the Secret Santa ones were, then we could think about maybe a limit of 1500-2000? Again, I'd like to hear what other ideas people have.
I'm not sure whether we had to write all aspects of the prompt. The way it worked last time was we filled the prompt, and we could draw ideas from the "what I like reading" part of the questionnaire the person filled out to help get other ideas if we wanted. For example, the prompt I got was "Johnlock fight that gets resolved," but in the "what I like reading" answer, "Viclock" was included in the list of things the person liked reading, so I used that to help me get an idea about what they could be fighting about. And for the one that was written for me, I said I liked platonic Sherlock and John, which the person wrote, and then the Christmas mystery prompt was the plot of the story.
I think how often you post the stories depends on how many there are and how long you want it to last.
As far as ratings of stories, I don't recall exactly how far we went with the Secret Santa. I know we didn't write sex, and that if anyone wanted to include those kinds of scenes they were told only to put them in the AO3 version of the story after the Fic Exchange was over. Everyone who could or wanted to posted their story on AO3 after all the authors had been revealed at the end and were grouped together, but I think that's optional. But it's nice to have them all filed together to look at again later, other than on the forum. I liked being able to bookmark mine.
Also, I don't know if you noticed, but for Secret Santa we had one topic for posting the stories and one for commenting on them. I think it kept things pretty well organized. It's up to you whether you want to do it for this one as well.