I would say most discus keepers have cardinals and I have never heard of discus eating cardinals.....I think the big issue mixing angels and discus is more the internal and external parasite issue. Angels often carry Capillaria and they usually resist it pretty well, whereas even a healthy discus will fall ill very quickly if exposed to Capillaria for instance.
This is an excellent point, and one that has made me shy away from mixing discus and angels. Also, I'd have to agree that the angels I have kept are much more aggressive at feeding time than my discus. That's just another reason I wouldn't personally mix them. My experience is that discus do very well with passive tetras, and I even keep mine with a breeding pair of apistogramma steindachneri with no aggression issues.
I think the most important advice I've ever received was that juvenile discus under 4 inches should not be grown out in community tanks. Unfortunately, I wasn't able to heed that advice when I was given 4 3 inch juvies for my planted 55 gallon community, as I had no grow out tank. Even with 50% water changes 3x weekly, all my original discus have died. I suspect a parasite is the cause, even though I treated the fish for parasites, however, there's nothing to say that it couldn't have been brought on by water quality issues, even with frequent water changes, as my 4 inch discus have done much better under a more lax water change schedule.
The world may never know, but one thing is for sure:
I will never again grow out small juvenile discus in anything but a 29 gallon barebottom tank.