The body of Famas is made of plastic.only the barrel and bipod are made of metal. Unfortunately,It is known as one of the most plasticy AEG.
Yes.You can mount a M203 with a rail attached at bottom of handguard.That is can be done for sure,but you will risk breaking it. You can go ahead and do that for photo shooting or display,but it is not very practical for field use.
Have you read this thread?
A very good review from Bob the Angry Potato
http://www.airsoftcanada.com/showthread.php?t=21658