fp.cfg is used only by FP-IDE.
No idea why you want to override its configuration file with fpc.cfg, because the FP-IDE offers you the possibility to customize its settings within the configuration options. You can add your own (additional) rules if you dislike current settings. Only realize that it keeps settings for different kind of releases (debug, normal and release), but you can add your custom rules to each one of them.
I didn't even know (so i doubt) it is possible to overrule the settings of FP-IDE which uses its own internal compiler. Because i doubt, i should really check that to make sure ...