E-Coat on new fenders

This depends on how well you trust the manufacturers ecoat process. If you trust that it is actually bonded to the metal then you can sand it with 320-400 red scuff and epoxy from there. If it were me I'd strip them to bare metal and epoxy them immediately.... then proceed from there. New modern sealers will actually stick to non sanded OEM ecoat. Before the manufacturers had such quality control I would clean new panels and paint them without any sanding or scuffing. They now come with so many imperfections that I sand all the outsides of the panels, red scuff the edges then seal and paint. I can guarantee you that OEM ecoat is stuck to the metal... aftermarket not so much and most paint manufacturers suggest you at least sand it with 320-400.

Also, I would highly recommend you check the panel fitment before assuming it's good and painting them. Most aftermarket sheet metal will take work to fit correctly and have good gaps.