The problem with cardinals is they are sold so small. My not overly successful attempts to breed them in numbers suggests they are sometimes under 3 months old in stores. Once they get to adult size, they would be safe with discus, but if they're wild caughts, they hatch around December and are sold in February/March, and farmed ones are really rushed to market. I had full sized wild caught scalare not eat any cardinals in a 75, but the cardinals were 2 years old. You almost have to have a cardinal growout tank, and add them as they get big enough. It would be like a large QT tank, but QTing from predators.
If you compare aquarium-common tetra growth with Cichlids, there's no contest. My Pristella maxillaris were the size of their parents in very little time. Their growth rates are turbocharged, which makes sense given the predators around them.
I don't heat my tanks except for breeding, so I look in the cool water direction. But there must be high bodied small tetras that like Discus habitats. They're surface fish, but hatchets?