I need your advise!

Will go from Florence to Rome for one day. Your recommendations will be greatly appreciated to maximize that day in Rome.

Thanks

It will depend on the day of the week. Some things are closed on certain days.

This is not the info you wanted but one day in Rome is not nearly enough. You need at least 4 days to thoroughly explore just the Coliseum, Forum, Palatine Hill, Pantheon and Trevi Fountain. That being said, I would recommend taking a walking tour because that will bring you by all of these sights so you will know where you want to spend more time the next time you go. You really speed through everything but it will provide you some history as well.

The Scala Reale walking tours get great reviews here: scalareale.org

If you have never been to Rome ( seems likely from your question) then my suggestion is to walk from the coloseum through palantine hill to Piazza Venezia and then up Via del Corsi. Take detours off this street to Piazza Navona, the Pantheon, the Trevi Fountain and the spanish steps.
This is a whirlwind walking tour but that is probably your best bet.
My second choice would be to go to the Vatican early in the am. Go to St. Peter's and the museum. This will take at least the morning. Take the subway to the Spanish steps and take the previously described walk in reverse order. Skip what you wish and end up at the coloseum. Have dinner in Trastavere and fall into bed!

I think you should visit St. Peter's Basilica in Vatican City to see the Pieta, Bernini Altar, and Michealangelo's dome. (I don't think you will have time to see Sistine Chapel, as there is usually a very long line to get in.) I would also recommend seeing the Colosseum and taking a stroll along the Tiber. Visit the Spanish Steps and Piazza di Spagna, stopping to have lunch at a cafe near the beautiful Piazza del Popolo and Borghese Gardens. I am one who thinks that the Trevi Fountain is overrun with tourists during the day; it is better seen at night.

I completely agree with Doug P. That's exactly the one day itinerary I would recommend (but one day in Rome seems criminal!) Check the Scala Reale site for a walking tour to help you shrink the city down a bit.