The Crimson playlist is there if you only want to play the Crimson map pack and nothing else. If all other players you are matched with in the regular playlists have the Crimson map pack, it'll still give you some options on those maps. In other words, you can still play on the Crimson maps while in the other playlists.
Thanks a lot for the info man, sorry for the late reply. finally finished the SP just now, good ending, quite emotional. this was a good game, to me as good as Halo 3 and much better than Reach.