Is the new switches don't come with the correct license, you usually have at least a 90 day trail period to get the right license install. If you are going to stack them, make sure both switches have the same IOS and the same license installed.
Yes, you can do everything before finalizing the license. Just make sure whatever license they come with is the same for both switches. When you install the finial licenses, you would have to reboot the switches in order for the new license to take effect but that can be done later on down the road.