First off, fish would be measured in cubic inches because they are three dimensional
There are two kinds of overstocking - keeping a fish in too small of a tank, and "overcrowding". A 15 gallon tank will encompass both types, and is not a good candidate for overstocking. Just about any school of fish you might keep ought to be kept in a larger tank, and adding most other fish with the school will make things over crowded. You might be able to responsibly get away with the smallest of the small.
Here's what I mean - say you have a school of 10 neons in a 90 gallon tank. Those 10 fish will only occupy about 10 gallons of water. So you can have several larger schools of fish in the tank because they will all occupy a different portion of the tank at any one time. Selection of fish is very important as to not have too many fish at any one level in the tank - they should be layered. So you see the problem with a 10 or 15 gallon tank - there is not room for multiple groups of fish without them being on top of one another.
Now for stocking larger solo/pair type fish - again a small tank just doesn't have the space. If you are stocking with the smaller schooling fish, then you want something that won't eat them. Such a fish generally won't care about the schools entering their space - they are more concerned about the other solo fish. Rather than visualize the fish in the tank, visualize the fishs personal space bubble as well, and that will help you determine how many of those kinds of fish you can have. A smaller fish like a Bolivian ram will have a smaller bubble. A larger fish like a severum will have a larger bubble. The important thing here is that are territories for each fish. The problem with a small tank like a 15 is that those bubbles will be overlapped most of the time, which leads to stress and confrontation. Small overlaps aren't usually such a big deal.
My rule of thumb for overstocking small schooling fish is one fish per gallon. A 29 gallon would be the smallest I would overstock. An example of how I've overstocked a 29 - 3 schools of fish (top, middle, bottom), a pleco, and a centerpiece fish/pair, putting me at around 30 fish or so in the tank. I have always managed my overstocked tanks (some as high as 200% according to aqadvisor) with big filtration systems and smart stocking. I don't do live plants - in my experience they are not necessary for overstocking, and in my opinion the plants can take up room the fish would otherwise need if taken to an extreme.
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