Overstocked is a relative term in my opinion. It all comes down to the type of fish, the types of filtration and maintenance, the division of territories in the tank (as in the plants, decor, rocks, etc) the feeding routine, et al.
Say you're keeping somewhat large, aggressive fish - well in that case you'll be stocking very sparse so they can have room for territory and reduce hostility. But if you're keeping say, cardinal tetras and corys you can stock the heck out of them since they never establish territory and have little to no aggression towards each other.
And when we talk about fish needing room to swim, is there really a definition or guideline for that? It's not like fish are walking on the ground in a two dimensional area; they are swimming in a three dimensional volume. Think about it: when you are walking somewhere and there is someone else coming the other way, each of you has only two choices of how to avoid hitting the other person. Two fishes swimming towards each other have more than eight directions in which to go in order to avoid the other fish.
This is not to say I would stock the tank the same way. I believe in having as little a bioload as possible for innumerable reasons. It all comes down to having a clean system.
Though I would never stock livebearers and acid loving fish together. It's a water chemistry thing. ;
