Rome or Florence customs

Immigration and customs are two separate processes. Immigration will occur when you get off the plane in Rome, they inspect your passport, allow you into the European Schengen Treaty Zone by stamping your passport. Customs will occur in Florence where you collect your baggage and declare any goods for which Italian import tax is owed. Generally, you just walk through a door marked "Nothing to Declare" and go on your way. This is assuming you are ticketed from the US/Canada through to Florence.

Just to be clear. If you are booked directly through to Florence via connecting flight in Rome then it will be immigrations in Rome and customs in Florence as previously stated. However, if you have two separate tickets first to Rome and then to Florence via a local airplane then both immigrations and customs will be in Rome. BUT customs is near an issue. Just walk through the green door - Nothing to Declare.

Are you flying Alitalia? That's the only airline that flies from Rome to Florence and they have only 3 flights a day. The Alitalia non stop from ORD arrives at FCO at 1:05pm. The Alitalia flight from FCO To FLR departs at 2:40pm. If you miss that flight, the next (and last for the day) departs at 9:30pm.
If you fly with American Airlines (the only other non stop to Rome), that flight arrives earlier (at 9:25 am) but you still have to wait for the same 2:40pm Alitalia flight to FLR because the earlier flight departs at 10:00 am and 35 min are likely insufficient for the immigration procedures and transfers to the domestic terminal 1.
I go to Florence early and occasionally have flown to FCO (including with AA from ORD). In those circumstances I would take the train from the airport to Roma Termini then from there to Florence. I have flown Alitalia once all the way to Florence (via FCO). It was a waste of time, that day the FLR airport was closed and they diverted me to Pisa instead. If I had taken the train I would have arrived many hours earlier. I never did that again. The train also offers more choices, basically at least 2 trains every hour instead of just 3 flights a day. The train ride from FCO to Rome Termini is 32 min. Then from Termini to Florence is a 90 min ride and the train drops you in Florence city center. So even allowing one hour for flight arrival procedures and 20 min for the train change in Rome, you can be in Florence within 4 hours from landing. That is faster than the Alitalia flight in most cases. If you fly Alitalia you land at 1:05pm. I'm sure you can be downtown Florence by 5pm with the train option. If you fly the 2:40 plane lands at FLR at 3:40. The best you can hope is getting to Florence downtown by 4:30pm. Not much difference. For sure you should not entertain the flight option if you fly American. In the 5 hour layover that you would have in that case (from 9:25am to 2:40pm) you can get to your hotel in Florence, have lunch and visit a museum even before the flight from Rome has landed at FLR at 3:40pm.