You can keep Discus (not discus fish !!!) in a community tank but the other fish have to be chosen with care. Most tetras will be ok with Discus as will angel fish and some of the less robust catfish.....corydorus, small plecos and otocinclus will be fine. A small shoal of harliquins would be a welcome addition.
Keep away from things like clown loach, livebearers, gouramies and anything that will nip (not all barbs nip !!!)) or is particularly boisterous.
You cannot get as many as you can afford as wildwood suggests, the recommended stocking levels for discus is one adult pair per 25 gallons of water....so a 48 x 15 x 18" tank should house no more than 2 pairs.
The tank should have plenty of cover in the form of plants and bogwood and heated to around 82 F. Water should match that of the dealers tank....if they keep theirs in tap water, you should have no problems either although soft acidic water is usually best.
If you are going to buy discus, try to get tank bred fish rather those that are imported....they will stand a far better chance of survival and make sure you ask to see them feeding before you buy....if they wont feed or the dealer wont let you see them feed....walk away.
Good luck