I would say the minimum for discus is either 1, 2, or 6+. A loner discus sometimes works, but depends on the fish. Having 6+ is ideal, but the trouble starts when 2 of your discus pair up. They will defend part of the tank as "theirs" and aggressively attack intruders. That leaves 4+ discus in a much more confined space. If you take the pair out (ideal scenario) and you only had 5 to begin with, you will have 3 remaining in your main tank. You can expect the weakest one to take a massive beatdown from the other two.
I wouldn't keep discus unless you are really experienced and have spare tanks available. Unless you know for sure that they are fully grown, they will require daily water changes to avoid stunting. 50% daily water changes for juveniles will still stunt them as adults. When they mature into young adults, it eases off a bit, but the first year is the challenge. They are very sensitive to their water so you can't just dump in meds like many other fish.
Discus often only accept live or frozen foods unless they've been fed dry foods since they were young. Its very difficult to introduce new foods, especially to adults. If they ever get sick, or just for the heck of it, they'll start a hunger strike.
If you are still set on discus, I would highly recommend getting a proven or unproven but friendly pair. Young adult pairs that have been fed dry foods is probably the best. Oto's will do OK, but prefer slightly colder temps. Its the same thing with cardinals, and some of the larger discus might try to eat them.
You should head on over to simplydiscus.com and read all the stickies to see if you're up for the hassle.
