Your tank is only as powerful as your equipment, and your consistency.-me
Granted, there are *some* rules to abide by.
1) Size of tank must be appropriate to fish.
ie. Don't stick a nurse shark in a 300g tank...
.. that's just cruel.
ie. A large tang can typically do well in a smaller tank then most would claim, but can stress much more easily due to varying tank quality.
2) The more fish, the more filtration is needed, as long as fish are appopriately sized, and appropriately tempered.
ie. You can have 40 firefish in a 25g tank, that's *way* beyond the 1"/5g 'rule', but because these fish are so peaceful and expend so little waste it works out fine.
3) Don't think a fish is 'un-keepable' just because everyone else says so.
ie. Green Mandarin - this fish could pretty much go in almost any tank with healthy liverock (Suggested: at least two large chunks completely covered in growth).
4) Established tanks require daily maintenance. The more stuff you have in your tank, the faster things can change. "Overpopulated"? Look at doing checkups generally twice a day.... unless you have pretty hardy fish, or you like living on the edge; cleaning grime off glass and syphoning sand.
Personal experience has also shown me that nature will quite clearly show you its borders. Look at my (sad) "kill" list below. That wasn't my responsibility, but its owners who had way too many fish. Granted not all of them were in the tank at once, but he kept trying to add more and more, and the tank just wasn't having it. Typically when you cross that natural 'border', you'll lose more than you just added -- as fish durability and tolerance will only stretch so far.