The two-hour time slot is a good figure if you're looking at leaving the plane to arriving at Termini. Our flight landed a few minutes early, at 6:15 pm instead of the scheduled 6:30 pm. It took about 45 min before our luggage arrived. Clearing customs was a breeze, like the previous poster said there are different lines for EU and non-EU originating countries.
There were two flights that landed at about the same time that were going through baggage claim. After the luggage started running through, people were staggered enough getting their things that we spent perhaps 30 seconds in line, and another 30 seconds getting our passports stamped. Granted, we may have been lucky, but I did notice that they weren't taking too much time with most people. Things may be different for you.
The Leonardo Express to Termini leaves every half hour.
Here's the link for the train.. If you are going to fly to Venezia, you may or may not be flying out of FCO, you may have to get over to Ciampino. That requires the F4 Metropolitan train out of Termini. I am not too familiar with the train service out that way since the Metro lines do not service the airports. It takes about 30 min to get to Termini. Realize that the Express arrives at Binario 28 or 29 and if you are trying to get to Venezia the same day, those sometimes leave out tracks that are not close by. Binario 29 is the track that is all the way in the far corner of the station, depending on how much luggage you have, it'll be several minutes to get to the other trains.