I'm not a discus expert, but wouldn't that setup be too stressful for the 3 discus? Especially in a 55 gallon?
Discus are a schooling species of Cichlid, a 55 gallon tank is a good setup for new Discus; This is because 90% of Discus owners get them as juveniles which would make sense for a 55 gallon tank. Discus only tend to reach 6" max in size in home aquaria, which leads back to my earlier comment.
You can actually fit 4 fully grown 6" adult Discus in a 55 gallon tank, and the only stress that's normally created to Discus in a non agressive tank are water changes, re-arranging the tank, and during spawning.
"I would go with 3 Discus, a school of 10-15 Cardinal Tetras, a school of 5 Corydora catfish, and 2 True Siamese Algae Eaters if it were my tank."
In this scenario ^, Cardinal Tetras are only interested in themselves and are too small to harass Discus, Corydora catfish are only interested in finding food at the bottom of the tank and darting to the top of the tank because they have a labyrinth organ that allows them to breath atmospheric oxygen. In fact, they must have access to oxygen from both the air and from the water (they also have gills) or they won't survive. Although corydoras are bottom dwellers you will frequently see them come to the surface for a gulp of air.
True Siamese algae eaters are mainly focused on trying to find food like algae or left over food from feedings, they tend to harass themselves when there's more than 1 in a tank, it's more of a play harassing sort of like say 2 puppies play fighting.
