The GMC307 will light up whichever A/P mode is active and you will see the current and next A/P mode displayed on the PFD so you really don't need a separate indicator light to signal that the A/P system is powered.
There are a couple of different ways to disconnect the A/P without de-powering it. You can wire a signal to a discrete input for A/P disconnect, or you can just tap one of the trim switches (this mode assumes you are running the trim through the A/P servos).
I wired the power to the A/P servos through a panel switch just as you are suggesting, but I have never had occasion to use that switch. It's on when I start up and it stays on whether I use the A/P or not.
Your mileage may vary.