You can rent scooters in both Paris and Rome. In a quick google search I found the following for Rome. www.vemrent.com/en . We have traveled to both city using scooters though we own ours. I have traveled and rented in other cities and it is great because they send the scooter directly to your hotel. The bummer is you have to still have a wheelchair to get you to the hotel. For both Rome and Paris, I would recommend renting a 4 wheel scooter not the lighter 3 wheel scooters. The cobblestones in Paris and Rome need the larger machines in order to get crossed streets easier. They are usually only a little more expensive per day and are well worth it. One thing I will emphasis is that just because you have a scooter doesn’t mean you can get in everywhere you want to. Make sure your hotel has no steps into the front door and no steps between the lobby and your room. We take this for granted in America but I have found that this is not the norm in Europe. Most major attractions have a special entrance for wheelchairs and scooters but it is not always obvious. A little research goes a long way.
Hope you have a great vacation! You are going to my two favorite cities.