Here is the thing. I have done some research on this lately as well. In my opinion, while this might be "true" on paper, life does not always play by the rule book. After raising fish for a long time (And don't get me wrong, I am no expert), I can tell you that I have made many mistakes, and also made many wise decisions as well. In my mistakes, I have kept fish with an "ideal" warm temperature with fish that prefer "ideal" cooler temps. Personally I believe that while the "correct" temps and water conditions do somewhat influence the survivor ability of a fish, I also believe that a secondary portion of this relies on the individual fish.
I have read many things about fish that are widely believed to be impossible or unlikely occur in my tanks. I currently have 4 different varieties of rainbowfish schooling together, have had a Gourami, a Bala Shark and a Denison Barb school with each other, and have had cardinals thrive in 68-70 degree water... You get the idea. Unless my tanks are living in the Twilight Zone, I think that some very uncommon things commonly happen in all of our tanks that do not naturally occur in the wild.
I am of the mind that what you are commonly told about fish, is mostly just a guideline. It is "ideal" for fish to live in these conditions, but it is not the only recipe for happy fish. I have had fish thrive in conditions that defy the rules on paper for years past the "recommended or suspected" life span. I do recommend that you follow these guidelines, but if you have your heart set on cories, and Discus, I personally think you should give it a try.
My wife has a saying for our tanks.... "Only the strong survive". She is not referring to poor conditions or hinting that I mistreat our fish, but instead she is pointing out that there are good and bad batches of fish out there. You can find a great batch of cories that are strong, and will do great in warm temperatures, and find another batch that is weaker and will not make it under conditions that are considered warm for the species.
I am sure my opinion will be very unpopular with many readers, but I am just speaking from my own experience. I think if you want discus, that you should give discus a try, and if cories are the other fish you desire, you should give them a go as well. My only advise would to be to buy small and see how they do. If you buy 3 cories, and they survive a few weeks, go back and buys his tankmates.
Those are my long 2 cents. Thanks for reading, if anyone made it this far...