Overstocking happens when the amount of ammonia produced by your fish is too great to be effectively removed by the bacteria in your filter. It depends on a number of factors including what kind of filter you have and how much waste certain fish produce. It can also depend on how territorial some fish are. Visit
AqAdvisor - Intelligent Freshwater Tropical Fish Aquarium Stocking Calculator and Aquarium Tank/Filter Advisor to help give you a good idea of how much of a bioload your tank can handle.
Not exactly... but kind of. You're right on with the territorialism. But, it's not ammonia that's the issue. A tank can grow a bacteria colony big enough to support more fish than you can put in it, literally. But, what you have to monitor is the end product of the nitrogen cycle, the nitrate. You can stock a planted tank heavier than a non planted tank, because plants use nitrates to grow. PWC's are the only way to remove nitrate, other than nitrate filters (well, there are nitrate filters), but you also remove hormones, toxons, etc. Aqadvisor is a horrible tool to stock a tank with. It can give you an approximate idea, but should never be used as a final stocking tool. There are too many factors. The guy that made this may find one thing, where a bunch of seasoned veterans may find the opposite. It all boils down to making sure the fish have adequate swimming room (that depends on each fish individually), the nitrate level of the tank, and the compatability of the fish in the tank.
As far as that stocking list, I would agree that it sounds ok. The hatchets, I've heard, are prone to jumping, so a lid would be a good idea. I would also bump the cories up to 6, though 5 would be ok. I would introduce them before the rams, but after the others. Rams stick towards the bottom as well, and others have had issues with cories and apistos, which are dwarf cichlids as well. Just make sure the rams have plenty of cave areas, and lots of hiding places to feel secure and break up the line of sight. This will be especially important when they're breeding.
I'll add that everything I've said is purely my opinion... so take it as you wish.