the FG just buy its self, would be make sure you have use defalt light off on the front tabe of the render globals. Also, it gets its basise off of the enviment color on the cam your rendering from. So make sure thats a gray value, probably about nutral gray will get you some nice renders from FG. I would set the FG to 200 to start with and increse that up if needed or you get splocthys.
I don't normaly use GI, so I'm not much help there. But yeah low photon counts probably.
Some pic's of the problems your having might be a help yeah.