The average USAF T-6 student arrives with anywhere from 20 to 100 hrs in light acft, in my experience anyhow. Some show up with hundreds or even thousands of hours (mostly guard guys) but that is pretty rare. The other NATO countries typically invest in more flight time for their students before they send them here, since just getting them here is pretty expensive and if they don't have what it takes, it's cheaper to wash them out before they even get to the US.
USAF students get an intro in the diamond, yes. They model that training after what the students will see at the main UPT bases, including stand-up emergency procedures training and the basic military environment they will be in. But it is still an introduction course, not really intended to be a rigorous screening program. There is *some* screening at every step along the way (if you are bad academically, you won't be selected, etc) but a bunch of flying prior to UPT isn't really part of the screening process.