I am by no means the most knowledgeable person, but I do know the PRC is integrated in such a way that you can take just part of the content, like custom classes. That's because most of the classes depend on custom feats, custom spellbooks for caster classes, etc. For that reason, the PRC is pretty much an all or nothing affair, though there are a number of switches that enable users to customize the content (for example, by turning certain features on or off).