I've been testing PrimFaces 3.0.1 and RichFaces 4.1.0.Final together in a Java EE 6 project with seam-faces recently. I haven't had problems so far.
However, seam-faces is just there as a dependency to work around a conversation related bug (see my previous posts). I haven't used any of the features of seam-faces yet.
PF and RF developers may claim that the two component libraries are highly compatible; such statements should be taken with grain of sault. Intercompatibility is never tested thoroughly enough by the two teams. Some incompatibilities and unexpected result can still occur - specially, if you use versions that use different versions of JQuery (case in point: PrimeFaces 2.2 & RifhFaces 3.3.3.Final).
I'd suggest not to mix or nest components from the two libraries too much. Choose one component library and stick to it as much as you can; only resort to using components from the other for taks you can in no way accomplish with the one you chose.