Paris, Nice, Switzerland

Realistically 8 days is only time for 2 of these 4 destinations.

Also, it would probably be easier to advise you if we had all pieces of your trip plans in one place -- for instance if you are coming from Italy perhaps going to Switzerland on the way to France rather than backtracking, ending up in Paris to take the train to London.
My best tip is to cut back your destinations by about a third, selecting the places that call out to your heart.

Thank you so much. I will probably have to cut down on the places to visit.

That's too many places !!

What time of year is trip. If it is not in july or august then cut Switzerland.. you likely wont see much of mountains as its likely to be cloudy on mountains anyways and I assume you aren't going to see the cities .

Rome to Paris to London.

From Paris take Eurostar to London.. book ticket early for best price.
Fly from Rome to Paris.. and fly into either Charles de Gaulle or Orly airports.. NOT Beauvais Airport ( Ryanair uses it and its not convenient.. especially for you with such a short trip)

You have another post where you inquire about visiting Rome, Florence, Pisa, and Venice. So, between Rome, Florence, Pisa, Venice, Paris, Nice, and Switzerland, where exactly do you want to go?

was planning to fly into Rome and visit Florence, Pisa, Venice, then fly to Paris and cover Paris, Nice & Switzerland (if possible). was looking for ideas to make it happen but seems like its going to be a challenge so will try to reduce the places to visit. thx for the feedback from all responses.

You also have to figure in your travel days and the train rides , Rome to Florence 2-3 hours , Florence to Pisa 45 minutes each way , Florence to Venice 2-3 hours . Paris to nice 6 hour train ride . Then once you are there trying to see the sites in the time frame . Like you said you may have to adjust your schedule. Either way enjoy your trip ,