i think that over stocking is a pretty relative term.
If you have healthy animals and constantly good water parameters with 35-40 fish in a 10g then you are fine...
how many people lightly stock a tank and still can't maintain good water quality?
I would agree that you have a lot of room in that tank for more fish, you can safely add about 20 more, but slowly, 5 now... wait a few weeks to get a fair reading on the water quality changes, figure how much more food you need to add to the tank, and most importantly monitor things!
i had a 40g cichlid tank a few years ago with about 30 fish (all 4"+) i did my water changes, and i didn't overfeed, never had an issue.
The amount of fish you can keep in a tank really depends on both your skill level and your dedication level, not the size of the glass box you are using.