No, you can not have adsense and ypn on the same page at the same time. You can rotate adsense and ypn so users can see one at a time.
Yes it will. You can not have two contextual ad providers at the same time, on the same page. You CAN have YPN ads on one page, and Adsense ads on another page. Just not on the same page. You can rotate them with PHPadsNew. This software has a specific section for YPN and Adsense, and for this specific rotation.