15g a discus is a pretty good rule. Do regular water changes on them, have it well planted (so they have somewhere to hide). I wouldn't recommend getting small ones as they tend to need perfect water (not that discus don't, they are just harder than others to look after). I would get some medium ones, say 3-4 inch in diameter. A pH of 6.5 is good. Make sure the tank is well cycled, I would hate to add them in too early and wake up and find them floating.
If you buy them in a group, they tend to have less fighting than if they are in groups of 3 or less, and you may find you get some pairs (and get the tank bug ).
If there's anything else just ask
Chris-