Cardinals resistant to NTD?!? (and other NTD Q's)?

Sorry, but I think you are over stressing. It's great that you are thinking about your fish to keep them healthy though. I have read that cardinal tetras can resist NTD but I am not completely sure. But, I wouldn't worry too much about it. I have kept fish for 14 years now (and always had at least a few neons) and I only had NTD show up once. Even then i only had 2 or 3 fish die. You should be fine as long as you take the dead fish out before the others start picking on it. The first thing you can do to prevent a NTD out break is not to buy any fish from a tank with fish that look unhealthy. If possible, use a quarantine tank to keep your new fish for a while (though i have never used quarantine tanks myself).