I had 5 in a 10 gallon before, and I found it was much, much better then having 3. With 5 in the sorority, the agressin was much less adn they got along wonderfully, but I'f found with 4 or 3 - 3 especially - that the agression tends to be a lot more, and I've had to separate them every time I've tried a 3-female sorority.
You *can* overstock your tank by a little bit - especially so if you have live plants - like an extra inch or two. The thing is you just have to keep up with regular water changes. Everything should be fine with 5 girls in a 10 if you do your regular water changes.