I've heard of people who do get away with keeping discus (or any fish) in situations which are not by the book or ideal. However, there are lots more cases of people whose fish just die when they try this.
IMO it's far better to set your tank up to give you the best possible chance of success (ie. healthy, big, beautiful discus) rather than in a way that you may just get away with if you're lucky - you don't want to be battling against it before youn even start, or end up with a bunch of stunted fish.
So, for tank size, i'd go for as big as possible.
With tank-mates, cardinal tetra are generally seen as being calming dither fish to discus. However, these are not really what i would think of as fast, flighty fish. Again, all the literature I've read states you shouldn't keep them with anything too fast and flight as the discus will get stressed by it. I've not tried it myself, because I don't see why anyone'd want to take the risk (admittedly, I'm not keen on danios!).
