echo,
I know pressure rating is not the issue here but you also have to consider it. As an engineer, we do take lots of consideration. Regarding the mating between FF to RF you can check this link:
http://www.gouldspumps.com/pom_0007.html
That will tell you some inputs about flanges. And if you know more than me about the ratings & sizes of that specific flange, that is fine by me since I have been working only with a maximum of 4″ pipe for the time being. 