For some reason I think the two people that are "with" each other are the same people.. but whatever..
Anyways, I just wanted to put some imput on the basic question at hand. I understand the purpose behind why you want to feed your predatory fish other fish, but maybe a fish meant for this would be better? On the other hand, isnt feeding live fish also a good way to kill the fish thats eating it? I guess I would rather (if I did feed my oscars live fish, which I dont) not feed them a fish thats beautiful and belongs in an aquarium of its own, like these tetras your talking about, but rather maybe some dinky feeder guppies, or rosy reds?
Basically these high quality cichlid pellets that most people feed are a balanced diet for them, and seem to do better on them, than if they were eating diseased and parasitic filled fish. Im not trying to really get in on this argument, but im giving you an option and some imput on what I think would be best for your fish.
Some people, like you, are into the natural selection of fish eating fish and so on.. but there are other choices out there than feeding fish that should have a chance of survival in its own aquarium. Because like you said, you mentioned about how they get these fish, and what the fish go through, or what have you, but than you buy them and purposely end their life.
Now as for your choices of fish together.. ive heard you can keep a JD and Oscar together, im not sure the temperments of the other cichlids you mentioned, but im sure, if its a cichlid, its predatory. Anyhow, I think you could keep a discus in their fine, even with the oscar. Discus are oddly shaped fish, and if you bought them large enough, the oscar wouldnt be able to eat them, however since you have already had your oscar in this tank for however long, it could possibly bully around the discus. I have had my oscars since they were an inch long, and I house them with SilverDollars and even giant danios, they dont pay them no mind, but if I tried to add another fish, I could kiss them goodbye now.
Anyways, im just disagreeing with feeding them beautiful tetras, feed them live if you want, but I bet if you tested whether they lived longer on high quality pellets over live fish, im pretty sure I know which one would live longer..