Sorry to hear about the rummynose..
Maybe just go with the Discus and catfish.. I had success w/ adult rummynose, but my Discus were not full grown at the time...
But if you really want to add tetras, here are a few potential options I've kept:
Moenhausia collettii. They grow bigger than rummynose and school often in the upper third of the water column, albeit at a slower pace than rummynose.. Not sure your water temp, but they're listed temp range is up to 81F.
Moenkhausia costae.. They don't school as much, but stick together in the upper third and will school occasionally. Listed temp range up to 82F.
Hyphessobrycon coeruleus. Don't school much at all, but stick together in the upper third like M. costae. Listed temp range up to 82F.
Hemigrammus rubrostriatus may be big enough.. They don't school much but stick together.. Temp range up to 82F.
Hyphessobrycon pyrrhonotus.. They don't school at all, but will fill the tank, each holding a small area making a nice display.. Temp range up to 82F.
One other option may be instead of adding tetras, go with some smaller growing, or dwarf cichlids.. Biotodoma cupido stay smaller and temp range up to 82.. And Mikrogeophagus ramirezi temp range is up to 86F.