The limiting factor is how long you take to clear immigration and get your bags, and into which terminal you are arriving. The size of the crowd really does not matter. If your flight does arrive close to on time at 0650 you should be able to make it by 0800, definitely by 0900. But no need to worry, as the RER runs often and you do not need to reserve a seat on a specific train. Get there when you get here, buy your ticket and hop on the next train which will likely be leaving in less than 30 minutes.