Missus and I did Paris to Venice night train last May, glad we did it once for the experience, won't do it again. Nothing drastically wrong with it except time, $'s, little sleep.
If you do decide train, book early as we (read I) procrastinated and had to do Paris/Nice/Venice in order to get a double sleeper compartment. Also had a 5 hour layover in Nice before catching the night train. Nothing wrong with that, we saw a bit of Nice and the views from the train at night along the shore of the Med as it headed to Italy were lovely with all the lights. That said, once was enough.
You might want to consider flying one way (Paris to Rome??) and do trains for Florence/Venice/Paris going home. As others have said and I agree, too much for 9 days even flying both ways. We visited only Paris, Venice, Rome in 11 days and felt a bit frazzled at the end of it.