US to Italy via Paris - Where is immigration/customs stop?

Am I crazy or do I indeed go through security/immigration/passport control in Paris then customs in Florence?

Yes, passport control/immigration at CDG because you are entering and going to a Schengen treaty country (Italy). Customs (usually the "green" exit) after picking up checked through bags (if any) on reaching Italy.

I don't know about flights to Florence, but I had a CDG experience flying from Prague via CDG to Oslo. Flight from Prague arrived at a remote CDG location where we were bussed from the aircraft to a remote passport control and security screening building, then bussed to our terminal for the OSL departure. Our 90 minute connect got us to our OSL flight minutes before door closing.

PS The fellow in front of us screaming at the workers about his short connection did nothing to hurry the process and I suspect slowed rather than speed up the process.

In my one experience of going outside to take a shuttle bus around the airport, which I consider a mistake, it was very slow, and you also have to go back in through security. Try an internal route if it looks at all feasible, for instance if you're transferring from terminal 2E to 2F, or similar. 1.5 hours should be enough, but don't dawdle. See also http://easycdg.com/ground-transportation/train-station-terminal2/.

If they have a later flight (the one you want to change to), then the good news is that they can put you on that flight, if you miss the connection due to flight delays or immigration delays, and if there is space available. On a different trip through CDG, that's the protection I had to rely upon when the immigration line took 45 minutes.

media,

I'm not sure why Delta would have told you that you'll be going through Passport control in Italy? From what I've seen after many trips to Europe that's always done at the first entry point to the Schengen area, especially as the flight you'll be using from Paris to Florence will be an internal flight and therefore no Passport screening will be done in Florence.

Paris / CDG is not the most "efficient" airport for flight changes, but if everything works well then 1.5 hours should be adequate. However, as Laura mentioned, don't dawdle! You should be able to research which terminals your flight from the U.S. will be arriving at, and which terminal your flight from Florence will be departing from. That will give you an idea of how you'll be getting from one terminal to another, and the time involved. It's difficult to estimate the time required, as that will likely depend on how many flights the Immigration officers are processing at the time your flight arrives.

One other point to consider is that if your flight from Paris to Florence is a code share flight with Delta and you miss the connecting flight, then it's their responsibility to get you there.

Some airports have a "quarantine" zone where you don't actually enter Schengen before going on, but that is usually when the airport you left and the one you are going to are both outside of Schengen. Then you are just "transiting through" Schengen. I would not expect it in this case.

If this is all on one ticket sold by Delta, then they are responsible for getting you to Florence. If you miss the scheduled flight to Florence, they should put you on the next one. When you are approaching CDG, tell the flight attendant about the connection. They might have a way to expedite you through immigration.

I don't see how you can not be going through immigration in Paris, even with one ticket, despite what Delta and another poster said. It goes against everything I've read, as well as every experience I've had. Every time I've flown into CDG and on to another Schengen destination, it's been on one ticket, and I've always gone through immigration at CDG. Customs is another story.

Your Delta flight is certainly in codeshare with AirFrance, since Delta and AirFrance are partners with SkyTeam.
From what I understand you will by flying to CDG on a Delta operated flight. From your nickname I deduce you will be flying probably from Cincinnati (CVG) presumably on Delta 228 directly to CDG.

Your flight, like all Delta flights from the US, will arrive at Terminal 2E.
Upon arrival you will go through 'passport control' (immigration). Customs (which is really just a walk through) will be at your final destination airport (Florence), therefore you will not see your luggage in Paris. Your luggage will transfer to your flight to Florence on an AirFrance operated flight to FLR.
Flights to FLR depart from terminal 2F. After you go through passport control, you will have to transfer from 2E to 2F. Be aware that terminal 2E consists actually of 3 separate terminals called Hall L, Hall M, Hall K. I don't know which one your flight will arrive at, since I've never flown to CDG from CVG (I've flown many times from SFO), however depending on which hall you arrive at, the options to transfer to 2F will change. From Hall K is a very short walk. From Hall L, it's a long walk (or a bus). From Hall M, which is a satellite in the middle of the tarmac, you need the underground people mover train called CDGVAL Navette. I've provided a link below for you to study the terminal maps and your transfer options.
http://easycdg.com/airport-guide/maps-terminal-paris-cdg-airport-charlesdegaulle/

One and 1/2 hours for a layover in Paris may or may not be enough for the transfer. Everything depends on whether there are flight delays, and on the queues at the passport control, and also which Hall of Terminal 2E your Delta flight arrives at. You just don't worry. I've missed so many connections to Florence in Paris that I can't even count. That is not a problem. There are 5 or 6 flights to FLR daily with AirFrance and your flight from the US is scheduled to arrive in the early morning. If your flight from the US arrives late, just go to the AirFrance transfer desk and they'll simply put you on the next flight to Florence. Years ago AirFrance even used to give you a coupon for a free meal and a free drink while you wait as a courtesy for the inconvenience of having missed your flight and being delayed a couple of hours till the next flight. Not sure if they still do it nowadays.