Interesting thread. I work in the fish room of a great LPS and I have to explain away myths about stocking rules daily. I always try to break it down this way to people: As fish grow linerally, their mass increases exponentially so the mass of a ten inch oscar is equal to that of about 100 white clouds. The more mass a fish has, the more waste it is going to produce. Allowing that a tank is big enough for a fish to turn around in and have at least 4 times its body length for swimming is one important part of making sure it is comfortable, but how much maintenace you need to do is dependant on how much waste is being produced.
If you choose to overstock a tank, you're going to have a lot more work on your hands when it comes to cleaning up after your charges. My 125 gallon tank is overstocked and I would never recommend to a beginner what I get away with in my own tanks, but I also do a lot of water testing and water changes and use live plants to help maintain nitrate levels.
I don't feel comfortable with any of these rules because the only one I can think of that applies to every sort of situation is that there are going to be exceptions to every rule.
