IME, don't worry too much about 'over stocking' with a few small fish according to the 1" per gallon rule.
If your tank is <3 months old then I would actually stock less than what the 1" per gallon rule allows.
If your tank is older than 6 months, then I wouldn't use that rule.
More important in my opinion is to over filter regardless of your stocking levels (more room for error), buy a test kit (or get your water checked at the lfs often) and stock slowly.
You should obviously have no Ammonia or nitrites, but if you can keep your nitrates ~20ppm with the normal weekly water changes and the fish are healthy with plenty of swimming space, then your tank is not over stocked. With nitrates under 20ppm, you can easily add more fish.
Always remember however that some fish produce much more waste than others, fish like goldfish and plecs are very messy so take this into consideration.
Some people believe that you should stop socking when the nitrates hit 40ppm, but I've read a paper (and heard from other sources) that stated there is an affect on fish once the nitrates go over 30ppm. However the tests were done on fish from fast flowing well oxygenated rivers so they would be used to lower nitrates. It's better to be safe anyway...
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Oh, and lots and lots easy fast growing plants help too, preferably low light plants so you can also have floating plants.