Well I can give you my opinion about this as I am dealing with this right now. I have a huge Cobalt Discus that is my baby and no I cannot take it to the store. Had him since he was very tiny, anyhow I do have to say that they do get lonely. I had an Angel in my tank with my Discus but now as they are full grown all they did was fight even though they were bought as tiny babies.
Today I just went and bought some Dwarf Blue Neon Rainbow fish to see if this will help him out with some other swimming fish instead of just the catfish in there. I have live plants and the tank looks very homey but they do better in groups I would have to agree. My tank is not big enough for a group of them and if I were to add another Discus he would tear it apart I am sure.
They are truly beautiful fish no doubt and one of my favorite. But then again there is no "law" saying you cannot just have one in your tank either just other Discus folks will not like it.
They do require very good water quality and dedication on that part so if you are not up on your water changes you might want to look at some other kinds of fish.
I would love to get a bigger tank oneday and devote that tank to a nice school of them as they are a room show stopper for sure.
If you happen to go with Discus be very careful what other fish you put in with it and do some homework before you buy.
Hope this helps.....
If you raise discus with neons/cardinals from a young age they will not see them as food. Also discus are schooling fish and need company, bigger tanks are always a must with discus. They like a lot of free swimming space.
Chris
Hey Chris, how do you find the loaches you have listed with Discus, do they bother them at all??