...get into Rome Termini 9:10am, next train that I will need to transfer to leaves 9:35am. Too tight to get off one find my next platform and get on?
25 minutes should be enough time to get to your long distance train from the Leonardo Express. Just keep in mind that the LE arrives at the extreme side of the station, and it can be a long walk to get to the area where you can get to your platform. You can find videos showing the walk on YouTube.
But I do agree with the poster above that it would be safer to allow approx. three hours to get to your departing train at Termini.
And in addition, Trenitalia Economy tickets will allow one change to date and departure time. So if you are seriously delayed, you could access the Trenitalia app and change to a later train - you would have to pay the difference in the fare. So best to create a Trenitalia account, and install the app on your phone. You could also make the change at the Trenitalia ticket office at FCO.
Note that Italo also offers changes to some of their fares, so it would also offer the flexibility to change to a later train if delayed, again paying the difference. And if your flight arrived early and you breezed through FCO, you might even be able to change to an earlier train.
Finally, you'll be arriving at one of the E gates at FCO - it can be a very long walk to get to immigration, baggage claim and the train station.
