People, please don't offer advise unless you are 100% about what you are talking about.
Discus and knife fish would be a no IMO. The knife fish are semi-nocturnal, and would spook the discus while they sleep, leading to stressed discus. Stressed discus usually start refusing to eat if the problem goes on long enough, and are renound for volenteraly starving to death if something isn't right. For this reason, I'd advise leaving the knife fish out of the planned discucs set-up.
Discus need a group, but not a 12 strong one. Five-Eight is the usual recomendation for group size for beginers. You want 10 gallons per fish, so you could get up-to seven, but the minimum tank size is 55g

I'd mebe get 6, leaving 10 inches of stocking for "diter fish", your compatible tetras.
To do well, the discus need lots of clean fresh water. Fish under four inches will need 50% waterchanges daily minimum, over four inches, they need at least 50% twice weekly, and adults need 50% weekly. Lots of clean water IMO is the key to healty discus

Not supplying enough clean water will stunt them, and is the most common discus "newbie" error.
Domestic discus aren't too fussy about water hardness and pH, but anything extreamly alkaline or hard should be avoided

A mature (six months or more old) tank is IME more important

Leave the wild caught discus untill you have experience with domestically bred ones, as the wild caught fish need exact water conditions that require lots of effort to maintain.
Best advise for discus, research, research and research some more untill you can answer all discus questions cropping up on the forum. When ready to get he fish, forget the research

Copy a working set-up and then add your fish to it. If it works, don't fix it. If it does not work, you will get a gut feeling from the research, which you should follow
HTH
Rabbut
Edit to add, you don't need 12 fish or a larger tank. Discus mix fine with angels so long as the angels do not pair
