i have read that they will not eat small fish because theur mouth is too smal, but other places hve told me that they may harrass the fish, but with plants and decorations to hide in and around that may not be a problem
It will work great!Only thing You may run into is,the Discus eating some of the tetras, when the Discus are older.Cardinals work great.They come from the same water parameters
I can't see a discus eating anything over 1/3". They're mouths are fairly small and I doubt they could even catch a tetra. Mine have trouble swimming after some brine shrimp swept away by the powerhead.