I would agree that discus are not that hard to keep- IF YOU KNOW WHAT YOU ARE DOING, and don't mind committing to the required maintenance. There is a difference between keeping healthy big long lived discus and keeping them alive for a while in less than ideal conditions.
I do weekly 50% water changes, and I dose the tanks really lean. I have difficulty with some plants showing deficiencies with water that soft, make sure you plan accordingly. Ammania Gracilis, R. Macranda and L. Cuba are some that I had to pull because that often show deficiencies in my tank.
If I don't do the weekly water change the discus show their displeasure by turning dark on me- you can tell they are not happy. You also have to keep the water warm, 84 degrees so mosses grow really stringy and ugly. I have also been exceptionally unsuccessfully with java ferns because of the low nitrates and high temps.
Your tap water will be fine for discus, nice and soft, but with a pH in the high fives out of the tap, its likely you have a lot of dissolved CO2 in the water. Let it outgas for a couple hours and test again to see if the assumption I am making is correct (ie the pH rises after the water sits for a while). If so, you might have issues with 50% water changes right out of the tap, its something to think about. You may want to do 2x25% or let the water sit for a while before adding it to the tank if possible, run an airstone, etc.
Its not just nitrates that you have to worry about. If that were the case, we could plant the tank to the hilt, and not worry about it, but Discus are sensitive to other dissolved organic compounds that naturally occur in the tank, so the water changes are a must.
You don't want to add different sized discus to the same tank. The big guys will beat up on the little guys and discus are sensitive to stress.
Growing out discus in a planted tank is not that easy. I have two sets of discus. One little bunch raised in a planted tank, who are only about 4" definitely stunted. I have another set that were raised in a bare bottomed 55 with daily water changes and fed 5x a day. The difference in the fish is NIGHT and DAY. I would invest in some nice big adult fish. If you have to go with 2" fish, and they have to go in a planted tank, I would suggest a sand foreground. They will pick up food from the sand, but aren't very successful when they try pulling food out from the gravel. They are messy eaters with bad aim, and they eat in mid water, so unless you are going to stand by the tank for 20 minutes 2-3x a day, dropping in a couple pellets at a time until they get their fill, I would recommend the sand foreground. With young fish you will have to feed more often to get them to grow to their fill potential, which also means you will have to water change more.
Keep the water warm, do your weekly water changes- every week, without fail, or more often if you want to raise young fish, feed them often and get plants that like to live in the right conditions for discus, and you should be fine.
I keep 4 discus in a 90. I think that is pretty close to max capacity. I have three females and one male. The pair spawns regularly in the tank, and the other two females are relatively docile, but they still pick on each other a bit. Be careful how many fish you add to the tank so they will have room to avoid too much conflict.
The plural of anecdote is not data