Since the SIM cards are "pre" activated by the scan activation in store, you should be able to use an adapter to activate them in another device. If they were completely virgin SIM cards, you would get the incompatible SIM card error if you tried to activate them for the first time in a device that used a different size SIM card.
Once a SIM card is already active on a line, you can use an adapter to switch b/t devices no problem.
I don't own any other Iphones, only one 6 plus and this phone still doesn't activate. This is One of ship to home disasters. I broke my leg and have not be able to get to the Apple Store yet.... huh! in a wheelchair...Still in 14 day return window. I decided not to mail it back just yet.