A few minor things to keep in mind...
You don't need to have the romapass in hand before calling to make the reservation. You can call and book the tickets as soon as your dates are available. You don't need to buy the romapass before getting to Rome. All you need to do is to call, book the reservation telling them you'll be using the pass and then show the pass when you check in for the tour.
The gallery only books about 60 days in advance so start checking the site around that time to see if your dates have opened. There is no need to call before that as the tickets won't be available.
Don't bring anything to the gallery that you are not comfortable checking. They will not allow any bags inside but will give you a clear plastic bag for your wallet and other small items. Get there "at least" 30 minutes early because that is when they start giving away unclaimed reservations. I like to get there at least 45 minutes early, check in, check my bags and then visit the bookstore and get a coffee. You can also stroll the ground while waiting for your entry time.
Donna