Mine aren't too jumpy unless the lights are off, or I close the top too hard while they're picking at the floating bloodworms! :P
They were a little jumpy at first, but they're full-grown adults in a spacious planted tank, so that would certainly account for them calming down.
If you're going to be getting smaller discus, especially in a tank of that size, I'd recommend 2 or 3 daily feedings of high quality flake or pellet and Hikari frozen bloodworms, and a temp. of around 86 to 88. Daily or bi-daily waterchanges would be best. This will ensure that when they are fully grown that they are absolutely the best Discus you could ever have.
Of course, you can take it a little easier--1 or 2 feedings a day of just higher quality dried foods and the occasional frozen bloodworms, and WCs every 2-3 days, though for growing out younger Discus WCs are a must.
When you're buying them, look for the signs of proper Discus shape and coloration--they should not be peppered or have an Angel-like mouth, where the mouth is fairly far away from the forehead and 'chin'. Actually, just look at my signature (bad angle in the avatar) for pictures of good Discus. :P
I wouldn't recommend buying them from a store if there is a breeder in your area--check out the Sponsors on
http://www.simplydiscus.com to see if there is one in your area. You may or may not be able to get them cheaper, but you can gaurantee that the fish you buy are healthy.