You'll need to keep a minimum of six discus of there will be trouble in your tank. If your buying small they will be fine until they grow. Depends what other fish you have in. I had six discus in my 5 foot (18 inches deep and high)). THey were find, but wouldn't have gone much smaller....
If your thoughtful about the decor and tank mates i think you could get off with it fine. Give lots of hiding places, but try not to crowd the tank. Plants work well - amazon swords etc.