If you are on time, you can probably make it, but do you have any idea what time the train from FCO to Termini is? They run frequently, but its still a 25 or so minute run, plus getting to the train station, and walking between trains at Termini. It might be a bit stressful
Immigration (called Passport check) and collecting your bags can take a while...plan on close to an hour. Customs is literally a yes/no question, and if you have nothing to declare is literally takes a couple of seconds to walk out the door.