Byron, I've heard completely opposite advice from multiple sources regarding the cardinals. I've heard that they're both very hardy and also that they're very sensitive. Once article I read compared them to neons and said that actually they are hardier than the neons which are domestically bred. The article concluded that since cardinals are wild caught, they are used to fluctuations of the environment as opposed to the neons who are brought up in controlled tanks. Interesting. My cardinals seem great and get along fine with the frogs and the betta.
The are many aspects of this hobby on which you will find varying opinions. Nothing wrong with that in many cases, but not all. You will find many advocating maintaining a Betta in one pint of water. You will find others saying anything less than a 2 gallon tank is cruel and inhumane. One must always consider the source of the information; some is obviously reliable, some is obviously not.
To the cardinal. These fish occur in very soft and acidic water. The water tests for their natural habitat streams always show hardness so low it cannot be measured, and pH varies depending upon the stream from 3.5 to 5.5, never (to the best of my memory) higher. Most aquarists find cardinals live 2-3 years, maybe 4. That is always because of the water. The great ichthyologist Dr. Jacques Gery wrote that he maintained cardinals for more than 10 years--but only in very soft, acidic water. Dr. Stanley Weitzman had articles in TFH during the 1990's, and in several he mentioned the effect of hardness on soft water fish. Dr. Hans Baensch in his Aquarium Atlas wrote of hard water causing calcium blockages of the kidneys in cardinals, and of the fish's "light phobia" as he termed it.
Fish have been programmed by nature over thousands and millions of years to suit their environment. Some fish seem to be able to adjust to differing water parameters better than others, over generations of being tank raised. All of the "neon" species [there are 3 regularly available, and a fourth was discovered a few years ago] are sensitive to water parameters, water conditions (ammonia, nitrite, nitrate), light and water movement. Neon tetra are commercially raised and have been for decades, yet they still retain this delicate sensitivity. They will absolutely do better in soft, acidic water, notwithstanding they have been raised for generations in basic harder water.
As for the fluctuations in nature, yes--but they are controlled over various periods of time, caused by environmental factors like the rainy season and the dry season, day and night (temperature and pH fluctuations occur every 24 hours). But never are these fish subjected to ammonia or nitrite above zero, and nitrates are rarely any higher either. Plus, relatively few fish in the enormous expanse of a stream is a very different environment from the confines of a closed system in an aquarium.
Appearance can be misleading too. Fish can look alright, but inwardly a calcium blockage is still causing trouble and ultimate death. People can have cancer and not know it--and I speak as someone who has been living with recurring cancer for the past four years. But it is doing its thing notwithstanding I outwardly show no signs of it.
I mentioned the source of an opinion. I have written most of the freshwater profiles here. To do so, I research highly respected authorities. And when I see such knowledgeable ichthyologists agreeing on this or that, I accept that it is most probably correct and worth listening to. Sometimes equally eminent scientists may differ; fine, I mention that when I see it. But you won't find many such cases, and I think that speaks volumes too.
Lastly there is personal experience, which should be worth something after 20+ years of fishkeeping. I have introduced cardinals to new tanks and lost the whole group within weeks; in established mature tanks, they settle in fine. Some say they don't care about light and filter flow, I say they do. In my 115g 5-foot tank I have a group of cardinals. The filter outflow is at one end so there is a gentle current down the tank, strongest at the left end under the outflow spigot, and scarcely moving at the right end 5 feet away. The cardinals always remain at the right end of the tank, and always under cover of plants except when they surface to feed. So do the rummynose tetra for that matter. This is not co-incidence; the fish when given the option will naturally choose what they are most comfortable with, and that is what is closest to their natural environment.