Solo traveler would like to do a day trip from Florence to
Rome and connect up with a tour in Rome. Possibility of an
overnight, but i am basing in Florence.
Not sure what the question is here. I guess everything. How to get there. What tour to take. Where to overnight.
I'll mention that the way to get there is high-speed train. It will get you to Rome very quickly. I guess whether you spend the night or not depends on how much you want to see. You can see quite a lot on a driving tour, but if you plan to get out and spend time at the sights, that really limits what you have time to see.
Better look at tours, decide which one covers what you want to see, then you'll know if you have time to get there, take the tour, and zip back to Florence all in the same day.
Think the high speed train is about an hour and a half. In the plaza in front of the train station is the starting point for a number of hop on/off buses. They will take you to the high points and provide some transportation. That is probably you most efficient use of a one day trip,