From personal experience, I'd say you have that almost exactly backwards. It's about 75% follow the rules, 25% what works in your tank. For example, I've found that just as we say here on the forum, it's best to overfilter and understock your tank unless you want to risk the health of your fish or your own health, doing all those water changes! However, as the exception to the rule, my singletail fish get along just fine with my oranda, and the general rule is not to mix fancies and ST goldfish. But it works in my tank.