Kind of a detailed question - am building up a bike and using a 36/50 compact crank, my builder has put on an Ultegra rear derailleur 6500 series. This is a new build, all Ultegra 10 componentry and I expected to see a 6600 rear derailleur. He's a good guy and we're both learning a lot as we proceed, but his supplier may have shipped him an older Ultegra part (6500 version as is like one on a 3 year old Giant bike I also own). He thinks there is an Ultegra 9 version and an Ultegra 10 version and one can use either one with no problem. This may be the case with Campy but I'm not sure about Shimano. I can use a 12-25 or a 12-27 rear cassette with the 36/50 and am wondering about the best rear der to use with Ultegra 10 system. Should it be the 6500 or 6600, or is it irrelevant as either one would be acceptable? Is there an Ultegra 9 rear der and an Ultegra 10 rear der? If I go with my 12-27 first choice for cassette does it matter which rear der I use? I am also assuming the triple version is not applicable to my build with the 36/50. Any info would be most helpful. Thanks in advance.