Cardinals are great discus food. They don't seem to eat rummy nose.
Keeping about 8 numbers of 3.5 inch discus fish in a 4x2xw feet aquarium was my plan. I have got about 50 cardinal tetras and 10 rummy nose all about 1 inch in quarantine. I was wondering if the discus would attack and swallow these cardinal tetras and if so will more drift wood help. It's going to be barebottom drift aquarium by the way.
Cardinals are great discus food. They don't seem to eat rummy nose.
At my age, everything is irritating.
I had smaller cardinals, and now have 2.5-3" cardinals ~10 in the same size tank as yours. They are doing well. Neons were expensive fish food, but the cardinals seem to be doing fine. My discus would rather eat vibra bites or Al FDBW at this point.
Put large Cardinal in first and let them get comfortable and find places where they can hide. After the cardinals get comfortable introduce small Discus. I've done it that way and it worked. If I were to try tetras again it would not be a variety so very soft and tender bodied.
Mama Bear