Yeah you can really do it either way...by adding it via the AT&T login or just swapping the sim card. However, when using the AT&T login method you describe, the ipad embedded sim (in the 9.7") is permanently locked to AT&T which sort of defeats the entire purpose of it. So I will just be swapping it back and forth. Plus this allows me to use it with my iPad mini when I don't need the full size ipad.
Mine is the 12.9" iPad Pro. I added my iPad 4th Generation to my regular family share plan AT&T. Although I do not use it much.