My picks for a 10 gallon would be 3 blackskirt tetras, 2 corydoras, and a male marigold swordtail. None of these get real big and contrary to popular belief, our two corys have been living happily together for about 2 months now. Everyone says at least 3 but I just don't see it. As for serpae tetras, we have 5 and they are just fine. They shoal together and never nip at our other fish. As I have said before, I will say again, fish DO have different personalities and other than putting say Jack Dempseys in with guppies which is just a senseless idea, you do have to practice a lot of trial and error. As long as you use a bit of commonsense and don't get huge or overly aggressive fish, you should do fine. Not sure what type of filtration you have but for more fish you need more filtration! You can overstock but ONLY with proper filtration!
Allie