Check behind the panel before removing the "knockouts". THere should be a space in the hotrail to insert another breaker. Sometimes the covers are also used on bigger boxes and not all the spaces are spaces-like the top of your panel. It says do not remove across the knockouts.
Having all your breakers add up to more than the main is OK. It is kinda like plumbing. The water source to your house is capable of, let's say 5 gallons/minute. Your bathtub faucet is capable of around 3 gallons/minute. NO problem. If you have 2 bathtub faucets, and you turn both of them on at once, the source can't keep up and the flow is reduced to both. Each will have around 2.5gallons/minute. Add the kitchen sink(around 1.5gallons/minute) and the amount available to the bathtub is reduced again.
To continue the analogy to your box--each circuit/faucet is limited to the rated amount but probably never even approaches it. (You don't always need 3 gallons/minute to wash your hands.)
I ramble--hope that makes sense.