As long as your reservations are linked you will be able to check in at the same time. You can find the check in date on the celebrity app under the profile icon. This is also where you check in. check in will begin at midnight embarkation port time. You can choose the arrival time at the port for all of you, at this time.
You are correct all you need is passport info and credit card info. It will also ask you to take a picture but you can skip this step since they always do it at the port.