Back in my discus days I often kept them with angelfish and other fish, including Raphael cats-- no corys, though, I had them as a kid and for me they were/are a been there, done that fish. Didn't know at the time recommended temps are a bit lower for the Raphael cats (seems to be up to 80) and I might not do it now without a little more research, but for me they were healthy, grew well, etc.
I never did daily water changes with discus (I did large water changes once or twice a week, depending on the setup), but I kept them in minimum 75 gal tanks with substrate and plants, didn't overfeed them, never fed beef heart, didn't have to medicate and deworm every so often like some people do, and over the course of raising several different groups I grew out nice, healthy beauties, some easily over 8 inches, and some of these would boss or co-boss tanks including Mesonauta and large angelfish. The more experienced I became with them the more I found discus to be robust fish if you get healthy strains and keep them sensibly. The only ones I'd have trouble with were emaciated wilds I'd buy to try to save them. I strongly believe it's the way some keep and feed them that requires fanatical water changes.
One thing I did find true that can affect stocking with discus is they can bicker with each other, especially while feeding, and there's less of it in larger groups, bigger tanks. Sometimes a smaller group will harass and stress a low status individual, not always ime but it can happen... it also seemed to be less when the discus weren't the dominant, or the only dominant, fish in the tank.
On a similar note, i didn't find my discus couldn't compete for food with other fish (like some people say), mine figured it out just fine.
