you did get somewhat bad information. I used to be on AT&T since the Cingular days. I just moved to Verizon in July but sim swapping always worked on AT&T, even if you had a device payment. However, the one thing to keep in mind is that AT&T's system sees Android vs iOS as slightly different entities. Meaning that, your data plan stays the same (amount of GB / $ / month) each time you make the sim swap, but their system has an iPhone data plan and a all other phone data plan. The data that you are subscribed to stays they same, they just have to make slight modifications in the system for everything to work correctly.
So, you are all setup on iPhone and everything is working as it should. You take that sim, pop into a Nexus (or any Android device) and everything will also work exactly as it should. Voice, SMS/MMS, data, etc. works just fine. But, if you put that sim back into the iPhone, some things like personal hotspot won't work until you call 611 and tell them to update the IMEI of the phone on your line. This is how it worked for me. I didn't always have to call when I was sim swapping, but noticed things got a little wonky from time to time going back from Android to iOS (without updating the IMEI with AT&T).
sorry to see you cancelled your Nexus order. This is a fabulous device. I do love iOS too and might pick up a 6s to tinker with but you are missing out on a great phone. If you do ever want to try the 5x (or any other android device), make sure you follow the instructions to degister your phone number from iMessage before you put your sim card into the android device. Also do the same on any iPads or Mac computers that have iMessage and FaceTime with your phone number enabled.
https://selfsolve.apple.com/deregister-imessage