I have had 14 different species of Synodontis over the years.
Synodontis are terretorial among their own kind so only one of any given type in a tank unless it is a) very large- 75 gallon or larger, and/or b) you have lots of hiding places for the synodontis. They will seem to get along okay at times but then they will get into a thrashing spell where they try to spear each other with their sharp pectoral fins.
They might tolerate discus temperatures but they are very active, flighty fish, particularly at night (they are primarily nocturnal) and they will probably spook the discus.
I don't remember if they will eat small tetras or not.
They are great tank mates for African cichlids, but a gamble in my opinion for discus.