Unlike Srixon and older Bridgestone forged irons, there doesn't seem be a flow of common design with Epon back faces. Each irons have unique looking back cavity design. Ideally, if I was to put a combo together, I'd want the back faces to be similar looking. More importantly, my thought would be to choose the rest of the set where the distance gaps flow with the long irons.
if the long irons are significantly strong lofted and multi material like the 503, you may end up needing two 6 irons etc... which wouldn't be ideal.