If you can get through Immigration fairly quickly and there is no delay with your bags then an hour and a half will be plenty.
Where are you flying from? I'm not sure what FCO is? If you're comming from a European country then immigration will be pretty fast but if you're comming from the US it might be slower as it's not an EU country.
If you're in the same terminal then it's really easy to get to the station. We flew in to CDG from Glasgow last year which is also in the same terminal and I think we had about 2hours and we had loads of time.
One thing I should mention though that I've encountered before is buying the tickets. If you arrive too close to the train departure time you can't buy the ticket at the automatic machines (tried to do that a couple of trips ago and it wouldn't register that train just the one after) and the queue for the actual ticket booths can be sooo long and sooo slow. The last trip therefore we ordered them online and got them sent to our house so we had them in advance - the only thing about that means that you have to book a specific train at a specific time. But alot of the tickets are transferrable so you can book onto another train i fyou are delayed, if the times are right for you.