I kept and bred many many many discus back in the day...and always used a 55G as a grow out on my potential breeders. IMO it is not a terrible sized tank for them...***IF*** you are diligent and have heavy filtration. I also see no reason why a shoal of fish would be considered a bad idea considering a shoal of small fish act as a dither and provide the discus with a sense of security. Fish like rummy nose tetras tend to loose the intensity of their red if parms go off...a good indicator to start working on WCs if their red is less than brilliant.

I will say there is no set rule to stocking fish of any sort discus included, do note that stocking itself is a balancing act between filtration...water volume...and water changes. A 4ft tank of a decent height offers enough room for 5 discus...I would stock no more. Filtration should be something you invest in...2 Fluval 406s or 2 Rena XP3s would be ideal. Canisters work best due to the fact Discus dont like a lot of current but like very clean aerated water. I would do 5 discus, 12 Rummy nose, 1 BN...no angels they tend to pester discus.

The best advice I can give you on discus is do not buy junk fish because they are cheap. Search out a good breeder of low F number domestic fish. Treat them and any other fish in the tank for a full realm of internal parasites in QT. Prazi Pro and Metro are you BFFs for life when dealing with discus. Your nitrates do need to be less than 5ppm...preferably zero. The water also needs to be soft for their best health. Some plant life in the tank can help you achieve this as well as using RO water if your tap isn't great. Get in the habit of doing multiple water-changes per week...a community tank of discus pushing the stocking limit should receive no less than 3 water changes per week of 40-50%.

Not a bad idea...just know what you are getting into beforehand.