What if I download a different IPSW for a different Device?
Nothing is going to happen. It won't take the IPSW file and give an error, something like:


  The iPhone "name" could not be restored


You should just get the iPhone 6s IPSW file. Another note: You can't get iOS 6 on an iPhone 6s. The lowest version is iOS 9 for iPhone 6s.