In the past I prepared a POWER8 build on a RHEL7 system. I think NethServer 7 can be installed on RHEL, but as you see there are some YUM repository issues. You need to install EPEL repository for additional dependencies: I don’t know how EPEL plays with RHEL. Some modules might require also SCL collections… In other words I’d expect some troubles when installing additional modules.
Supporting a “7Server” redirect to “7” on mirrorlist could fix some nethserver-install issues. However, before implementing it I must be sure it is worth the effort, and everything else works.
You can work around it by editing the NethServer.repo file: replace $releasever with 7.
…but why RHEL? Can I ask why don’t you install CentOS?