I am also trying to get it done...
As not mention, I am assuming the call flow will be something like Flash Client (video) -> FMS -> FMG -> Sip -> Polycom device (Sip Video).
Not sure about the FMS side coding. But in FMG you need to add H264 codec line in Sip gateway profile. I was able to make video call by creating 2 sip profiles in FMG.
Note :- FMG support H264 codec only. So make sure the sip client support H264 codec (CounterPath Bria ) softphone are working.