Hi Melissa
The CSV will make a variable flow pump out of your 10 GPM pump. So instead of the pump always producing 10 GPM, and cycling on and off while you are using water, the CSV makes the pump match the amount being used and the pump runs continuously while any faucet is on.
Cycling on and off destroys every component in your pump system. The check valve has apparently slammed shut so many times from cycling until it failed. Now your pump is coming on even when you are not using any water. This is why your electric bill is so high. The electric bill from a pump, when being used just for the house, should be no mre than about 5 bucks a month. Since the pump is running a minute and twenty seconds then staying off for only 40 seconds, it is running for 16 hours a day even though you are not using any water.
Replacing the check valve so the pump only comes on when you use water is the only way to get the electric bill under control. Adding a CSV would have saved energy by not letting your check valve be destroyed. Once you get the check valve repaired, the electric bill will only be 5 bucks a month, and the CSV won't reduce the electric bill any further.
The CSV works fine with any size tank. You just don't need a very large tank because of how the CSV works, but a larger tank won't hurt anything.