I've used the ST80 to guide my C11 successfully @ F/10, so the focal length is definitely not an issue.. (as a matter of fact, I've seen a 12" F/10 SCT guided by a 200mm camera lens...)
If you mount the ST80 directly to the top of the ED80, do your best to line make sure that with the focuser extended to where the QHY5/Orion guider comes to focus, your ST80 is around center to the ED80. (with the focuser extended to where your camera comes to focus)
This will make balancing the two easier..
I could not think of a better setup to start imaging.. ED80, ST80, EQ6.. It sounds very workable to me
