I've kept discus with angels and i keep in mind the following:
1. If its your first time to keep discus, dont mix them with any other tank mates until your confident enough to deal with any problems that may arise while your still learning. (My first time was a costly disaster)
2. They might have a difficult time competing with angels when it comes to food, if you have juviniles (discus), wait a few months, grow them out a little bit until the time that they are no longer that nervous when you approach the tank and you have spent a reasonable time observing each and everyone eats well and all your discus are growing at the same rate (differences in growth rate should be minimal).
3. Make sure you have a big tank to keep discus and angels together.
4. Make sure you quarantine the angels or any other tank mates you plan to add. Get them from someone you know well enough and has a good reputation in keeping them.
5. Discus can be teritorial since your adding angels on a latter date please make sure you check the tank more freaquently in the first few days for any signs of aggression coming from the discus (though rarely, it can happen).
6. Minimum of 3 discus in one tank (4-5 would be better).
I think those myths are in actuallity experiences of people who have kept these beautiful fishes. Experiences vary depending on different situations we have encountered and how we handled them. The pointers I mentioned above are the ones I encountered during the years that I have kept them, and you may notice that some of them are the basics of fish keeping.
Goodluck.