1. Without entering any codes or making any changes, I inserted my Straight Talk sim card and the phone booted up and switched on to the AT&T network without any prompts. In order to get data working, I had to enter the Straight Talk APN manually, at which point I was able to use 4G data.
2. I then removed the Straight Talk sim and rebooted the phone with a T-Mobile sim card. This time the phone prompted me for an unlock code, which I entered from a website that I had paid for an unlock code. The unlock was successful and I was again able to make calls on the T-Mobile network (I did not test data since I had no credit).
So for those of you who are like me and use Straight Talk as your monthly service, you should be able to use at least some of the Straight Talk SIM in the SM-G900A without modification. If you are also like me a travel out of the country routinely, you shouldn't have any problems buying an unlock code online right away in order to use local sim cards when you are away from home.
Hope this is helpful to someone!
PS Does anyone know how to make new APN profiles persistent? I noticed that when I removed the Straight Talk sim and put in the T-Mobile SIM that the Straight Talk APN I had entered went away, even when I put the Straight Talk sim back in. It would be nice to have the Straight Talk SIM permanently on the phone (or on the sim card?) so I don't have to input new settings every time I swap sims.