re these fish ok with Discus?
Depends. First, the discus need a group of at least five or six, so considering their mature size you are pretty well filling a 300 liter (70-ish gallon) tank.
Discus must have warmth, so that limits a lot of other possibles. Cardinal tetras do work, in a group of 12-15 here. Rummy nose tetras also do well in warmer tanks, and will not nip the discus either. A group of at least 15, up to 21 here if you decide on rummys, they need more.
I never recommend more than one neotropic species of cichlid in the same tank, with a very few exceptions. With discus, I wouldn't.
African Butterfly Fish are basically stand-alone species. They will eat, or attempt to eat, any fish approaching the surface, and as most fish approach the surface to feed, this is significant. And they have enormous mouths when you see them open. I had a male/female pair for many years, but in their own tank.
Coli already explained why dwarf gourami are a real problem, but gourami and discus do not work together. Cichlids and gourami generally are too much alike and should not be combined, with a very few exceptions in larger tanks.
The nitrate is still a real issue at 40ppm. Do you have nitrate in the tap water on its own?