Actually, I personally think that's backwards. I find cardinals to be much hardier. Neon Tetras have been so overbred these last few years that it's not unusual for you to lose quite a few from a new school, because they are weak. I hope that's not the case with yours, but should you have any issues with them, definitely let us know and we'll see how we can help.
75: blood parrots, featherfin cat, emperor tetras, turquoise rainbowfish, BN plecos, japanese trapdoor; 46: WCMM, gold inca snails, ghost, bamboo & amano shrimp, kuhli loaches, rummynose & ember tetras, endlers, platies, flame gourami, guppies; 16: pygmy, peppered, loxozonus corys, otos, assassins, RCS ... ~ Research PRIOR to purchase.... ~