Hi Darren,
I have neither FCS or SCE running (I'm a SCOM 2007 Admin) and have no personal experience so will probably not be able to walk you through things that crop up in your average dual install of these - this is about the sum total of advice I can give you but I figure something is better than nothing, yes?
Forefront client security uses MOM 2005 agent, this is a different service communicating on a different port to that used by the System Center version of the software. They'll also be reporting back to different management servers - SCE agent will be reporting back to your SCE server whereas the FCS will be reporting back to the Forefront system.
The respective agents should happily coexist side by side on the same system as long as the necessary pre-requisite software is in place but as with anything, testing the command line and the order of installation would be a very good idea (personally I'd go the older FCS before the newer SCE agent but you may not have the option - just in terms of their age, there's no chance of you overlaying old versions of files on new) .
Good Luck with this!