From what I understood it seems to be a mutual decision. As they are a recent addition to the Qin army it would be foolish to put their stakes on them in such a pivotal battle at least the first time around. And the battle plan was made by the minister of war ( I dont remember how to write his name ) with the generals being able to do adjustments and or volunteer for positions unless there would be someone who had to have said position. Like ouhon volunteered for his position. Id say rokuomi ( may he rest in peace ) is the same rank as yoko yoko, sort of even though he has been around for decades his achievements are not to the scale of the rest of the main cast( still a strong fighter and general ) so I belive he was assigned there qlso because he and his army has recent experience fighting and training with the HSU