1) They're only fragile if you don't meet their requirements.... it would be fairly poor evolutionary thing if they really did succumb to diseases as easily as some older literature suggests, after all. Give them exactly what they need and they're as tough as any other fish. What they need is a bit more challenging than some though - calm tank, clean water, etc.
2) Well, no fish will thrive if it's tank mates aren't chosen with care.....perhaps nothing is a "community fish" in the way some books would seem to imply. Discus are only trickier in that they need higher than average tropical temperatures, cleaner-than-average water, and need calm tankmates. I keep mine with cardinal tetras and cories.
3) H'mmm - I think if you were to look at examples of these "thrown together" tanks then you'd find that the ones that did well were the ones (either by accident or design) that had pretty closely compatible fish in it. So a "mixed commmunity tank" of discus, tetras, hatchet-fish, types of cory which don't mind the heat, maybe other small cichlids like rams... Or, you'd find the tank hadn't been going for more than a very few months.
4) to (7) Re: angel fish - I'd agree with other posters that the parasite could be erradicted by treating the fish in quarantine before being added in with the discus. The temperatures are fine. The temparament - more questionable: it probably does depend on the individual fish, the ages they are introduced to the tank, etc etc. And maybe a bit of luck. Am not much good on angels, though...so definitely recomend listening to someone else about them !