Some of these generals will die and be replaced by the 3 youngsters then. Ousen appears to be planning a coup, so he will need to be removed down the road for sure.
I predict that the sixth great general will be either Kisui who defected to Qin or Juko'ou who was captured by Tou in the Juuko arc. I am hoping it is Kisui since the author has been hyping his abilities and power since his first appearance.
I'm guessing the sixth is Shou Hei Kun, since central planning loses its importance when every Great General gets to do as he pleases. It's easy to forget but he isn't just a strategist.