I agree with the previous reply except I would also shave another day off Venice, it is a small, walkable town. Splurge on the three-day vaparetto pass and use it as bus and tour combined (Rick has a great audio guide for this). You can easily see all the highlights in one day but why rush. Plan to stay out late into the evening and get up early to see Venice like a local.
You need two whole days but with the TVG trains, you can easily get to Florence the evening of your second day. Florence is also walkable and mostly safe after dark if staying in the town center.
Siena is small and walkable but very hilly, as is San G. I did the "Siena, San G, small Tuscan vineyard" tour and wished for a little more time in the towns and less at the vineyard. If there are a few of you traveling together rent a car and choose two stops.
10/8 Venice (2-2.5)
10/11 Florence (3-4) - day trip to Siena, San G, Montepulciano, etc. If taking a tour look for English only
10/15 via Pisa to Cinque Terre (2-3) - stop for a few hours in Pisa on your way to Cinque Terre (or skip and do Amalfi Coast instead)
10/17 Rome (3-4) - you could add a day trip from Rome to Naples/Pompeii or Sorrento/Capri
10/20 Depart
I'll be in Rome and Florence at the same time as you but this is my second trip and I'll be spending my cliff-side sunshine days in Sorrento/Amalfi (better weather in October). My biggest piece of advice is avoid shopping. It is such a time sucker and touristy. If you find something unique on your travels pick it up but spend your time and money on experiences rather than things. This time around I am splurging on the "Breakfast with the Pope" tour hosted by Viator. It's a 6am breakfast at the Pinecone Courtyard and early entry to the Vatican museum...totally worth it to avoid the crowds.
I am an EXPERT traveler traveling alone so this rigorous schedule is totally doable for me...not recommended for the novice traveler.
10/5 Portland Flight from PDX to Paris
10/6 PDX/Paris
10/7 Paris
10/8 Paris
10/9 Reims
10/10 Rome
10/11 Rome
10/12 Florence
10/13 Naples/Sorrento
10/14 Positano/Amalfi/Ravello
10/15 Paestum/Sorrento
10/16 Sorrento/Capri
10/17 Pompeii/Sorrento
10/18 FCO/PDX
Happy to offer additional advice.