Would i have to unlock an AT&T iPhone 5 before using it on the AT&T GoPhone?

Would i have to unlock an AT&T iPhone 5 before using it on the AT&T GoPhone?

No, but AT&T does audits of their GoPhone network, they can see what type of phone is on the network due to their ESN… They will block the iPhone from the network when they see it.

Not at all. It's fine as long as the carrier is same.