Hi what size tank is it? In all honesty the volume of catfish we see in the hobby is so fractional its quite stupid.
In the smallest of tanks, you have the 3 or so species of Pygmy Cories and then as the tanks get bigger so do the Cories and a school of them always looks good
Then from there you get upto Porthole and Flagtail Cats which are relatives of the Cories which get to around 3 inches and then up from them are the Hoplos which range from 4 - 7 inches depending on species.
Then as Verminator says the Pimelodus family is a great family with Pictus getting to around 15 cm - though I could believe 20cm. And a few other species that are findable including the much larger P.Ornatus which is pretty massive. Pims are all predatory so I would not risk them with your tetras especially your Cardinals.
There are the Doras, which are things like your talking catfish but also include a few other smaller species like the nice Mouse Catfish which after seeing them in a shop I really really want as they look incredible and are really nice and active
But expensive!
Wood Catfish are a nice group as well and in my oppinion a really under valued family - though they are starting to come more common and in particular the smaller species which is great! Lots of nice species in there. I have some Agenisous in my tank which look great and always out in the open
Then of course you have all of the fancy plecos and the variety of sizes and patterns, lots and lots of nice ones out there
I really like whiptail cats though, IME much more active than regular plecs and some really really nice looking ones
The main thing with what your after is the Discus and their higher than average temperature requirements. So it would take a bit of research to work out which is best for this tank.
Wills