A lot of things are considered common knowledge because someone "wrote" it somewhere. I've found that you can learn a lot more by doing something yourself, instead of just believing what someone wrote =)
The tank's not overstocked. I use high grade carbon and zeolite (I cut open the Tetra standard cartridges and replace their crap carbon with bio-chem zorb, hehe). When I decided to keep this many fish, I bought test kits and started to monitor Nitrate, Nitrite, Ammonia, and PH regularly.
I'm really, really happy with this tank. All of the Bettas are PetCo rescues, four of which were dying when I bought them, and I nursed them back to health in the tank. There was a lot of nipping in the beginning, but I've watched that carefully, and all of my fish are showing healthy regrowth.
I R pleased. So R dur fishies.