Haha, yeah. I don't see a problem doing both, actually. Have the giant guide and just copy/paste the relevant sections into the other forums as well. That way you still have the comprehensive guide, but people can look at a more bite-size version as they work on each game individually.
That's not a bad idea of all and I semi-considered doing that before writing up the whole thing, as it would be a very easy way for members to view information in a less cluttered way, but I ended deciding against it simply due to laziness of having to write four more fleshed out roadmaps as opposed to the giant general one I came up with, lol.
What I tried doing in the Roadmap - above the overview and walkthrough - was to put a "Useful Links" section with links to each of the six distinct parts of the guide where new info was: Overall, Halo: CE, Halo 2, Halo 2 MP, Halo 3, and Halo 4. So if someone wanted to work on a skull from Halo 2, they'd click the link and it would bring them to the Halo 2 post part of the guide.
Though after the first day or two, if people are saying anything and asking for separate guides in the sub-forums, I will definitely adapt quickly and create four individual guides if necessary
Suggestion for your guide. Have you thought about posting the individual game achievements in their respective forums, sort of as their own guide (could even give each a road map of their own), and just linking to them from the main guide? I think that would kind of limit the shock of having such a huge guide, and make it easier for people to read and work on. Since those don't cross over at all, they wouldn't have to click back and forth when they are working on a specific game/achievement.