Hi Chris,
I would not recommend using a Gen-X as a replacement on that skimmer. That pump uses a different type of needlewheel altogether. While it does pump move water, it pulls less air and creates less foam. I would recommend going back to the Sedra3500 as the replacement. That will power the skimmer the best. Or you could even try and upgrade to the Sedra5000 with te appropriate needlewheel.
Thanks,
Jeremy