You can overstock a tank, I just dont suggest doing it in the first year. You want to give the tank plenty of time to mature. Plus you cant just toss any fish together and expect them to all get along. If you have mainly aggressive fish, dont over stock because each of those fish will require a greater amount of territory and they will defend it. They just dont like having a lot of neighbors. But if you have mostly peaceful fish, it will work out much better.
Finally, its really important to pick fish that occupy different areas of the tank. If all of your fish are perching fish, you will have issues because they will fight over those areas. If all you have are open water swimmers, you will have issues.
So yes, you can overstock a tank, but it does take planning and patience but mostly importantly, knowing your fish.