the way I read the thread so far you asked 'what size tank for 5/6 discus & 2/3 Angels and a ghost knife?'
Paul responded with 'The combo mentioned is fine in a 90 Gal'
If I read it all right then I agree with Paul. If you want additional fish get a small group of fish that are bottom dwellers. Angel and discus are wonderful beeautiful fish and all stay in the middle of the tank. So add a few bottom dwellers and make as much of your decor stay low in the tank to free up the middle for your fish. If your considering doing a planted tank I'd suggest floating plants.