There is no basis for treating the 3GHs, 6GGs and 7FDs as separate tiers. The lesser Fire Dragons might not be on par, but the better ones certainly are, and the 6GGs have handed the 3GHs historic, all time devastating defeats and more Ls than vice-versa. Ultimately they're titles with appointments that are more political than meritorious in nature, same as "regular" Great Generals, and the needs of the state differ from state to state. In respect of shared borders and political landscapes, it always made sense to me Wei had more "special" GGs than Qin, and Qin more than Zhao.
As for Ko Chou, he could credibly have joined the ranks of any of them.
In terms of comps, his closest is Go Kei, who remains of the most dangerous minds and efficient butchers in the series to date.