While the YN565EX is more powerful, you'll probably need to treat it like it's not the more powerful flash.
When you approach flash lighting, you want to think about the "key" light... this is the primary source of lighting that illuminates your subjects and it dominates the lighting scene. The rest of the lights are supplemental and provide fill, accents, etc. and you don't want them to dominate. They tend to be set at a weaker power level.
HOWEVER... you typically want that key light to not be ON the camera. Unfortunately the 430EX II can only work in a multi-flash setup if it is a remote/slave light. It cannot be the master. This means if you plan to use ETTL then you have to leave the 565EX on camera and have the 430EX off camera in order to use ETTL. But you probably don't want the "key" (more powerful) light on camera because that doesn't make for very interesting lighting.
You may have to put both of them off-camera and use manual mode or use the 70D's pop-up flags as a trigger (or buy a trigger). It is actually possible to use the on-camera flash only as a trigger and have it not appear in the final image. The ETTL system fires pre-flashes which serve both to allow the camera to test the lighting and compute the final flash power but ALSO to send instructions to the remote flashes. When the pop-up flash is up but in the disabled setting, it actually fires all the pre-flashes to send orders to the other flashes... but sends out one final flash to order the off-camera lights to fire a fraction of a second later... the shutter opens.. and then ONLY the remote lights fire. This all happens so quickly that you'd swear the flashes only fired once (when in reality they may have fired several times.) When you check the final image, you'll will not see any light contributed by the on-camera flash (that flash was only firing to send commands when the shutter was still closed.)
As for the size of the backdrop... I have no idea how large these subjects and costumes are going to be. A 12' wide backdrop certainly sounds reasonable but not having seen the costumes and poses... I'll leave that to someone else to comment.