Actually, Spartan137, I think that, ideally, the author of each of the substories could finish each of thier stories separately, and then do some hard-core editing during which they could intertwine each character of group's story-lines into each other, creating one complex, yet fluid, story-line.
And as a final note... I noticed you have a cover. Well, I came across a site that specializes in extremely small book publishing runs.
Cool find, although I believe that The Stig plans to publish this solely as an eBook (or PDF), and possibly submit it to The Source if it turns out good enough.
As for the length of the stories, I can tell you this: Today Stig and I (unofficially) finished writing and editting the first chapter installment of his own story, which focuses (as you can tell from the teaser) on a single squad of ODST's. We even made the chapter into a PDF file, with a front cover, etc. Just to see how it would turn out, and it still needs a lot of work. For nor, Stig is probably more interested in finishing the story than worrying what we have to do once the story is done.
I would post a link to the .pdf, but I wouldn't do that unless The Stig gave me permission.
As of now, I guess I'm sort of the Stig's unofficial editor, although I also plan to begin work on my own story which I will hopefully be able to intertwine with The Stig's ...