Customs in Rome

It is Passport Control that takes time, unless you are suspected of carrying contraband or have something to declare. Lines can be long, or not. I have transited Rome Passport Control in less than an hour every time.

Are you on a single ticket from ATL to FCO to PMO? If so, you should have an easier time of making this connection than if you are on two separate tickets.

Customs will be done in Palermo. Passport Control/Immigration is done in Rome. (They are two different things.) In most cases, customs is just a walk through unless you have something to declare.

If this is on one ticket, and you have checked bags, they should be checked through to Palermo. If two tickets, you will have to retrieve your bags in Rome and recheck them.

Try to get your boarding passes online to save time in Rome. If one ticket, this shouldn't be a problem if you want to get them at ATL.

Yes, as mentioned, one itinerary, or the Rome-Palermo flight booked separately? What airlines? Checked luggage? All play into how much time it will take. A couple hours is usually OK, but if you start adding other things, that time slips away quickly.

The true answer is nobody knows. If there are no delays (weather, mechanical, landing spot, and more) and you deplane without delay, and there are no long lines at passport control (say, several large planes land at once), then that should be enough time. But as you can see, there are many stages in your journey where a delay can occur. You may also have to go through a security check right after deplaning.

If you have you have separate tickets, you can sometimes get checked luggage tagged through to PMO. If you can't (check with your ATL-FCO airline) you should take only carry-on. Be sure to check the cabin luggage limits on the FCO-PMO flight. If you have too much, you may be able to gate-check luggage. You want to avoid having to collect luggage at FCO which puts you landside and means re-checking the bag and going through another security check.

If you are on a single ticket ATL-FCO-PMO, are there other flights to PMO that day on the same airline? If so, you will probably get on a subsequent flight.

Thanks for your responses. We are flying Delta to Rome (arrive 11:05a). Then we fly ITA Airways to Palermo. I paid slightly more so I believe we can take our carry-on suitcases (Rick Steves' rolling ones) and our small backpacks on board the ITA Airways flight at 1:15p. We have no checked luggage. I think I can check us in on the ITA Airways flight 48hrs beforehand. We are arriving in Palermo 2-3 days before our RS Sicily Tour.

When we flew 2019 via Rome to Palermo, Delta to AirItalia (at the time) there was a special passport control area for transiting passengers near the arrival gates. It was quick.

I did this itinerary in May with separate tickets and it was very easy.
And we had checked bags (but 4 hours between flights so there was not need to rush).
You will have to walk over to the ITA (domestic) terminal but its a quick 10 minutes if so much.
We were on AA and then ITA.

When we transited through Rome there was a fast immigration check just for transiting passengers. We only waited a minute or two.

Sounds like you are well-organized and probably a significant flight delay would be your only worry.

Some tips I've found useful for tight connections. . . .
Tell your flight crew. They may be able expedite deplaning. Choose seats near the front of the plane.
If there's a long line at security (if there is any) or at passport control, at some airports there is a line for short connection passengers, at some airports a staff person will get you to the front of the line.
Use the toilet shortly before landing - you may not have time in the terminal.