if we go to Florence from Rome for one day only is it enough time to see everything with a private tour guide
This is one of those it depends questions. It depends on how broadly you define everything. You certainly can walk pass everything but seeing is another question. If you have one day for Florence, then you have one day and see what you can. You can always see more the next time. We were there three days and there are still things that we want to see on the next trip.
you can get only a brief glimpse of Florence in a day, for some people that is enough or a good start
I personally love Florence and need much more time to enjoy the city
We did Florence as a day trip the first time, and as others have said, it depends what your expectations are. We were able to hit the Academia, but not the Uffizi; saw the Duomo, but did not climb the Dome, wandered the streets, went to the market, shopped on Ponte Vecchio, had lunch. There certainly were other things to see, and on a second trip spent the night and saw more. Having a guide may help, maybe more efficient use of your time, they may have access times to the museums, and basically keep you on pace.
My parents and I went to Florence in 2010 on our first European trip. We were there for the day and saw the Accademia, need reservation to enter. Walked the bridge over the Arno, went to the Duomo, walked to the Piazza with the copy of David, went into the church there, did a little shopping. That was enough for us. Honestly...we didn't like it at all.