The only major difficulty in my experience doing EPEL/Fedora builds from the same source has normally been the prohibition on using SCLs in EPEL, which at least

The only major difficulty in my experience doing EPEL/Fedora builds from the same source has normally been the prohibition on using SCLs in EPEL, which at least has been relaxed recently to allow devtoolset on EPEL. You can build for EPEL6 just fine on RHEL7, assuming you have the privilege to run mock. (EPEL packages are currently built on Fedora 27.)