Well, I would look at it like this, this is your first outing with Discus, so to prevent that turning to tragedy I would play it safe. A lot of people are turned off Discus because the first experience is bad and they lose a few fish. Most people lose a few neons too, but they arent particualrly known for being 'difficult'. Thats probably because of the high price of Discus, and the usual stuff you hear about how hard it is to keep them.
So, for the plan:
For a tank of that size, I would get 6 or 7 smallish 4" Discus from someone who really knows what they are doing. Make sure the tank has been running for over 4 months at least with some tetras and a few catfish. Gibbiceps plecs are quite large and make a lot of waste, so it will build up the filter well. I wouldnt advise keeping him with the Discus though as he might like the taste of their sides, so make sure he has somewhere to go when the Discus arrive. Some small apistos like the Cacatuoides like the same water and are a better choice of bottom scavenging fish than corys IMO, who may not like the heat/depth or oxygen content of a Discus tank.
I dont advise Angels and Discus together, unless they are adult Discus and juvenile Angels, and in a very large tank. One or two pairs of each would be ok for your tank, but Angels are real bullies towards fish of the same size. They may keep the Discus from feeding properly and you may not see the Discus as much because they wont be as secure. For best results, dont mix them IMO.
Congos are too busy for a Discus tank IMO.
The rummy noses are a great choice for a Discus tank, and I really like the red flame tetra too (the red phantom is a bit of a pest).
If I were doing it, which I will be shorlty

, then I would keep other fish to a bare minimum until the Discus are there for a month or so, you could keep the other fishin quarantine for that period, and add them slowly.
The bottom line is that Discus do not like surprises in any way, shape or form. Whether that is new fish, a different pH, temperature, surroundings, rap music etc etc... they just dont cope well with anything out of the ordinary.
Another point to mention is that Discus dont like medications, so if you do plan on keeping other fish, the discus welfare, rather than the sick fish will have to be the priority, so a treatment tank is a must. The other fish will have to like the same conditions as the Discus, so the Discus are the main focus around which everything else should be judged. I would keep Discus at no lower than 28C or 84F.
If you can afford it, a UV steriliser is a great preventative measure.
Good luck
Ken