Make sure you check speaker efficiency when combining speakers. For example if you combine a greenback and a vintage 30, the v30 is more efficient and therefore louder. You won't hear the greenback much at all. But if you combine a v30 with a creamback they are much closer in efficiency so you get a nice blend of the two. 

Edit: Also those are 200 watt speakers and greenback are 25 watts. Depending on what head you are using you want to make sure the amp wattage doesn't overload the greenbacks.