Yes, it does have to float. I will put a lot of this down soon, and it will be glued at the joints with RTV. I rescue dachshunds, and sometimes the rescue will leave me a present, so sealed joints will help me a lot, LOL!
6.5 Creedmore for sure! I have a 16 penny nailer, but it is for 2x4s etc. I will be adding 2x2s with it so I can put 2 inch foam on all the walls to raise my R value, to lower my energy use.