1 word of advice. Buy all your discus at the same time. Never add a discus to your already established tank with out putting the new fish in quarintine for at least a month. I would shop around and get exactly what you want then enjoy but don't add new discus. I'd do at least 6 to help spread out the pecking order and give them clean water. Good luck. If you do cardinal tetras get them at the same time you buy your discus too.
If you just buy small ones I would get 8 as buy time they are big enough to over crowd the tank a couple may have already died off. Or if you did get a breeding pair you could sell them for quite alot $$$
Some one had a tank on this site with about 8 Discus's with an Arowana in also it looked great the Discus's were about 4/5" and the Arowana was about 10/15" what a site man ! it looked great i think they also had a Leopoldi Ray too.