I kept angels with my discus, and a lot of other stuff too including a dwarf pike, and never found any problems mixing them with other fish as long as I was sensible (no oscars!). I never found discus nervous either, I suspect a lot of that is a hangup from keeping them in bare bottom, bare tank setups where they are very nervous. The deal with big water changes somes from breeders - I once went to a discus farm in Malaysia where they were keeping them at insanely high stocking rates as this is where the young are most comfortable, and they were on an overflow system with continual water changes. I would say tho' that discus do seem to be senitive to water quality, and dissolved organics, so I did like to do big water changes on mine, and they responded well to it, even tho' I was overfiltered on that setup.
The diesease stuff is just bull, actively unproven I believe.