We land on a September Friday morning at 07:25 at FCO. Am trying to guesstimate what train I should book ex-Termini to head south. Do u think
we could realistically make at 09:45train or should we wait until 10:45am?? anu input would be appreciated alot!

0945 should be doable but tight connection at Termini.
Arrive 0725, immigration, baggage, customs and walk to train station (in FCO)
Depart FCO on 0908 train ('cause you'll never make the 0808 train and arrive Termini 0938.
If you are comfortable making the change in 7 minutes, then go for it. Doable, but risky if you have a lot of bags and you rely on the train being on time. We are talking Rome here, not ZRH or FRA

Will you have checked bags? If so, then be safe and take the 10:45 train. Bags can take a while to arrive - esp. first thing in the am b/c of the number of TATL flights that all arrive between 7-8 am.

In you aren't checking bags, you could probably make it - but it will require everything going very smoothly - starting w/your flight landing on time.

Two weeks a go on our first trip to Italy we landed at FCO just after 7:00 am and easily made the 9:45 am train. We may have been lucky at their was no line for customs and baggage came in about 20 minutes.

There are at least 3 factors you will have to take into consideration, 1) will your flight arrive on time??? During the summer months (or really at any time) you could have delays on take off, which would then alter your arrival time. 2) as others have mentioned, will you be checking luggage?--and then have to wait for it to arrive, before going thru customs 3) the train from the airport to Termini arrives at the far end of the terminal in Rome and it can be quite a walk to the main part of the station. That said, I would go for the 9:45am train and if you miss it, just change your ticket for the next one. Worst case, you won't be able to get on the next one, and might have to wait for an available train. In that case, store your bags and walk around Rome until your train's time.

A couple of considerations:

#1 - if you purchased an Amica fare, changing your train is not an option
#2 - during all of August and on Sept. weekends, the trains are very full thus the waiting for an available train may prove to be several hours

I would add one other factor no one's mentioned.. the variable of how long the lines will be for Passport Control/Immigration. I've never encountered a line or wait for Customs (just walked right past them) however I have been on the Passport Control line for up to 30 minutes (that was definitely the worse). I've always arrived in the A.M. coming from either JFK or ORD, the Non-EU lines have always seemed longer and the quickest I've gotten through it has been about 15 minutes. (I've done that route about 12 times now in the past few years).

Purchase your Train tickets to the South at FCO to save time