I believe the Wolf Tooth Road Link will work with your derailleur. It just attaches to the present dr mount, then the dr mounts to it. All it really does is keep the chain further from the cassette to give more room for larger cogs. I live in flat land, I need no more than a 46 front, ever. The 34 rear cog I never use here locally, but when I travel to areas where some climbing is necessary, it is there. Now, my Soma Smoothie with the 46-38 and 11-40 is going to be my traveling bike. When I ride locally, unless it is very windy, I can ride that as a 1X in the big ring. Going into a strong wind, it pretty much becomes a 1X in the small ring. I am really going back to simplifying my 2 wheel world. "Multi-Use" bikes are what works best for me at this point.
I think 6600 rear dr came in only short and medium cage. That may be true for the road groups that came after 6600. And I certainly could be wrong about that. I googled and found info that states both the short and medium cage DR have the same capacity listed, but lots of info on that being stretched. What I really know is what I tried, and works, on my bikes. There are almost always going to be different approaches to get the gearing that is wanted, at least real close to it. Some of what I have done was simply a matter of wanting to try, but not wanting spend dollars to do that. If it does not work well, it's a learning experience, or not.