wow, looks great, no that I see your substrate, I seriously advise against discus at all. you would never be able to keep the nitrates down low enough for them not to stunt with that substrate.
as far as a centerpiece fish have you looked into any of the dwarf south American cichlids. I small group (5 or 6) of a smaller cichlid would really look good and do not require the high maintenance that discus do. a couple options for this tank that are plant friendly would be: (my opinions only)
cupid cichlid (biodotoma cupido)
red breast acara (laetacara dorsigera ) I have a group of 6 of these and they are extremely personable.
bandit cichlid (guinacara geayi)
golden dwarf cichlid (nannacara anomala)
bolivian rams (mikrogeophagus altispinosus)