Hi love_fish
Overcrowding (overstocking) can mean either of those two things, but it usually refers to having more fish in a tank than can be supported by the beneficial bacteria with weekly water changes.
What happens if you have enough filtration to keep the fish is that the end product of the waste conversion will be nitrate. This is harmless in small amounts, but harmful if there is too much. This can be controlled, to a certain extent, by extra water changes.
Corys contribute no more to the bio load than any other fish of their size that eat the same amount of food.
IMHO, if you have 33" of fish, calculated by using their adult measurement, in a 29 gallon tank, you will have no problem at all unless you have high nitrates naturally in your tap water. If this is the case, you will have a hard time keeping the nitrates at a reasonable level and you might even want to understock to keep things manageable.
As for having too many fish on one level of the tank, it usually means that it will just appear out of balance, but, depending on the kind of fish you keep, it could also result in aggression or other stress on the fish. This is never good.
The inch per gallon rule is just a guideline. Depending on your experience and the amount of time you have to spend working on your tank, you can usually keep more fish than that.