Uh oh- that's not a good mix. That Synod catfish will eat the Cardinals just as soon as he's big enough to fit them in his mouth. He'll grab them at night while they're sleeping. I lost a whole school of Black Neons that way once.
They have been tank mates for almost 3 years now. The Cardinals are 3-5 years old and started out in my first planted tank. The cat is at least 4 years old and came from a relatives tank. He is around 6.5" and the cardinals are 1.5" - 2".
I havent had any problems with him except when I brought home a Bichir who wanted to be sociable. He couldn't learn to stay our of the cats stump and had to go back to the store.
...of course...there are those stories of cardinals\neons that start going missing after years in the same community so I wont say it will never happen.
However, for now I have to keep him because he keeps my wife content. She doesn't understand why I insist on keeping "tiny fish".