Hey
If I only need a 8A ESC, but get a 20A will this work?
cheers

Yes it will work. The 20 would stay cooler than the 8, but if your motor is only rated to lets say 8 amp and you push more than that, or enough to fry the motor, the esc would not shut down to protect the motor. You could burn the motor and esc.
With that said if you know your within the specs of the motor and battery, nothing wrong with using a larger esc, it will help with the heat and current flow. BG