For your arrival date, I see a train as late as 20:55 (8:55PM) from Roma Termini that arrives at Fir

Background: We arrive in Rome from the US at 4:30 pm on Feb. 22 and planned to go directly to Rome via non-stop train (or at least that is what I thought). My husband tells me that we can't make the connection vacation tour packages and italy with customs (or if our flight arrives late) because the last train leaves at 7:30.
I have no idea what you mean about "the connection with customs." Unless you are arriving vacation tour packages and italy in Rome with something to declare at Customs, it's not more than a 2-second issue. Of course, you will have to go through passport control/immigration, which can take a few minutes.
For your arrival date, I see a train as late as 20:55 (8:55PM) from Roma Termini that arrives at Firenze SMN at 22:22. There is an even later train, but it goes to Firenze CM, a station that may not be very convenient, taking more than 3 hours to do so.
I understand getting to Termini using the Express. I was confused as to the train schedule from Termini to Florence's SMN. My husband thought we wouldn't be able to make the connection from our arrival in Rome to departure from Termini. We want to be sure to leave ourselves enough time.
Will you be checking luggage? That is the real unknown time in the process. I've always had good luck with checked luggage at FCO, but I've heard horror stories of long waits for luggage vacation tour packages and italy from a few people. My last time with checked luggage I was buying train tickets about 50 minutes after arrival time, but I was familiar with the airport.
Let's consider the worst case: If the last train to Florence leaves Termini at 20:55, you might just make the connection on the LE that departed vacation tour packages and italy at 20:08 (8:08PM), vacation tour packages and italy though I would not recommend this. Better to choose the 19:38 (7:39PM). As you can see, that leaves you 3 hours between estimated arrival time and last LE possibility.
If I am reading the ticket policy correctly on the trenitalia website correctly. If you purchase a ticket in advance, it looks like you can change your departure time once between the time of purchase, and one hour after the train's scheduled departure - perhaps that doesn't include a departure time for the next day, depending on how you read it.
Yes, you should wait and buy tickets for the Leonardo (14 ). There is no advantage to buying them in advance (no discounts/seats). The Leonardo is an R train that doesn't have any reserved seating so doesn't sell out. You must validate your train ticket before boarding (yellow box track side).
There are other options besides the Trenitalia train from Termini. You can use the new Italo Rail from either vacation tour packages and italy Rome Ostiense or Rome Tiburtina. To reach either of these from the airport, you take the local FR1 train (8 ) which departs every 15 minutes (not the Leonardo). Ostiense is closest, but offers less trains (last one at 19:10). Tiburtina has trains at 19:25,19:55 and 20:55. Travel time from FCO to Ostiense is 30 minutes vacation tour packages and italy and FCO/Tiburtina is 48 minutes.
Depending on how many planes have arrived from abroad, immigration at FCO can take a long time. We waited in line an hour last time (about the same time of day), worrying about making our connecting flight.