The Yonkou are definitely more powerful than any Admiral. Both Whitebeard and Shanks showed this when they both stopped Sakazuki in his tracks during the Marineford War. As for Doflamingo, I think Luffy has to make a strong showing against him considering Kaido is, almost certainly, the next villain. Kaido should be the fight that Luffy struggles with, not DD. I think DD and Luffy will be on fairly even footing until Luffy pulls something out that gives him an absolute advantage, whether that's Gear Fourth, more powerful Conqueror's Haki, or perhaps something else.
Personally, I'm guessing the D. is going to play an important role in this fight, especially considering the revelation that Law is one, the revelation that DD is a disgraced Celestial Dragon, and that the D.'s are diametrically opposed to the Celestial Dragons. As for DD surviving to fight another day, it's possible, but there's really only two ways. Either he'll relent and join Luffy as an ally, or he'll run back to the World Government for protection. Somehow, though, I feel DD may be the first person Luffy kills. It's a long shot, but DD has seriously pissed off Luffy, and the D.'s may, instinctively, feel the urge to wipe out the Celestial Dragons.
We should know soon as Dressrosa will definitely finish by chapter 800. It's going to be an epic final 20 chapters!