I dont think it makes much difference on the bacteria colony, the more fish in the tank the more bacteria will build up and eat the ammonia. So an over stocked fish tank that is being up keeped with frequent water changes wil have no affect on the bacteria colony.
But there is a fine line between over stocked and stocked fully. For example my 10 gal has 2 Bronze corys, 3 Kuhlis and 8 Neons. Some people will say my tank is over stocked, but i do frequent water changes and keep up with the demand of these fish. I personally think this tank may be a bit under stocked because the fish mentioned are not large and are mostly bottom dwellers. So i have room for more Top to middle strata swimming fish. For example a molly or guppy.
Now a 10gal that has a few Bala sharks around 2" is overstocked. Because they have a heavy bio load and will get bigger and require a much larger tank than a 10 gal. Even if the owner were to perfrom frequent water changes they still would have to re home the fish because they couldnt live in a 10 gal tank.
Do i make sense?