Here is an article that I think is highly relevant to "overstocking" (Edit: when it comes to planted tanks):
http://www.barrreport.com/fish-plant...ish-loads.html
The aquarium in that article is much more heavily stocked than most tanks, even in proportion, and yet the fish are perfectly healthy and happy.
lopez, you stated you are doing 50% water changes weekly already, so this might be an interesting article regarding stocking for fish tanks in general. It's based on the EI method fertilization, which ties in well with weekly 50% water changes.
I don't mean you should stock as heavily as that tank in the article, but it just shows you that there are many possibilities you can do when it comes to fish stocking.