They are not generally ff approved. I recently sold one James made me in 2005 that he said should be ok for ff. I made a skinny Xcel for it and shot it without any problems for the past year. My new one that I got this year has never had anything but a skinny Xcel string on it. No issues whatsoever. I also have one James made for me about 10 years ago that I would not put ff on. Just depends...
A skinny Xcel string bumps the speed about 6 fps but if you're worried about it just use B-50. They're great shooting bows either way!