One inch is the rule, but you have three things to consider. Type of fish, where they like to be in the tank, and bioload.
One inch of goldfish or pleco for example is a bad rule of thumb since they produce a lot of waste and can grow to be large and you wouldn't want a one 6 inch fish in a 10 gallon tank.
The other thing to think of is where the fish "live" in the tank. Do they stay at the top, middle, or bottom? If you divide up your variety of fish based on where they like to be, they will have more living space.
And lastly bioload. If you have an appropriate filter (not an undergravel), your tank is fully cycled/established, and you keep up on weekly water changes then you are more likely to have success with an overstocked tank.
Plants help keep a tank healthy, but do require some work. There is a product called Algone which is basically a plant in a pouch. I use it to have the benefits of a planted tank without all the work. I also mix artificial and live plants to fool the eye, it's very hard to tell.
I am overstocked. I have 17 beings in a 12 gallon. The rasboras stay more at the top, the danios/white clouds in the middle, the corys at the bottom, the snail and shrimp are all over. Everyone has their space, and without fail I change the water once a week.