I tried to use "gem" to install "mongrel", but I got the error "can't find header files for ruby." I installed the ruby-devel RPM, and the gem installation works. It looks like ruby-devel is required for proper rubygems operation, but RPM did not complain about any missing ruby-devel RPM when I installed rubygems.
No, a non-devel package should never require a -devel package. The issue is that rubygems is fundamentally broken when it comes to binary gems. One way to work around that is to build those gems into rpm's (e.g., using gem2rpm) mongrel is also packaged as an RPM; if you just install rubygem-mongrel, all will be fine.