The 1" per gallon rule is really just a stocking guide. It really depends on several factors. The fish, some (goldfish, plecos, oscars) are messier than others and produce more waste meaning that you can't put as many in a tank. Filtrations, more is always better as long as the flow isn't too strong and stresses the fish. Tank size, some small fish like danios that would easily "fit" in a 10 gallon tank won't do well in it because they are very active and need lots of swimming room. If you are a beginner, I would suggest understocking at the start. It's much easier to keep the water parameters where they need to be.
If your tank is only 10 gallon, you really won't be able to put much in it other than a few small fish like tetras, rasboras or corys. I wouldn't suggest guppies, mollies or platys since they are live bearers and you would have fry very soon and your tank would be over populated.