All of the skimmers that I've seen so far have 2" ports. I don't know, but there may be other sizes.
What if the suction for the main drains are supposed to be 2 1/2"? Would you install the 2 1/2" for the drains and reduce it at the skimmer port, and also run the 2 1/2" from the pump and reduce it at the skimmer?
Wouldn't you be choking the line off that way?