Should work fine, I just did a similar swap with no major problems. Big difference is the pump output, the ER modded version has a piece of slip schedule 80 coming out of it, the GenX has threads, and a little smaller inside diameter. I ended up opening the output up a bit on mine by drilling it out to get a little more flow, and used a slip fitting on the threaded output to connect it back to the skimmer.
Jonathan...