Like Debbie I have several Rick Steves' tours under my belt. I've done 2 solo and 3 with family members. For my/our first International trip in decades my brother/SIL, their 2 20-something sons and I chose Heart of Italy. I loved this tour! It was a great introduction to Italy and such fun. There was one solo woman on that tour and she mingled with everyone and seemed to have a great time. I love the division of group time along with independent time on Rick's tours and it works great on this tour.
One excellent thing about Rick, his tour guides and his/their mission is that they work on teaching you travel skills. For those that have traveled a lot this is no big deal. For those that live in a big city with public transportation this may not be a big deal. For folks like me who have never lived where there is a metro, bus or train system, this education was huge for me! It really helped to have the guide do a bus lesson our first night in Rome on the way to our walking tour. The next day after the morning with the group we were able to zip around like we knew what we were doing! I was especially surprised that my nephews looked like they had taken subways for years. At the end in Florence she took everyone who wanted a lesson in buying train tickets to the train station and helped them get tickets for their next destination.
I am also entirely comfortable as a solo traveler on any of Rick's tours. The groups are usually made up of people who are interesting, active and welcoming. On the 21 Day BOE I did solo I drifted from group to group and had a wonderful time. Sometimes I spent time with the other solo travelers. Sometimes I spent time with other couples. Sometimes I spent time solo if I had something I wanted to do/see. Sometimes the whole group spent free time together. (I am thinking balcony, wine, Switzerland!!) I also never ate alone unless it was my choice. I am FB friends with maybe a dozen people from that 21 day tour. I would travel with that entire group again in a heartbeat! But then, I think most of the groups are like that!
FWIW, I do have some suggestions for people traveling solo on Rick's tours in my Trip Report if you care to look at it.
Overwhelmingly I have seen people post experiences like Debbie and I both seemed to have. Occasionally someone will post that they did not enjoy being a solo on a trip and it is good that they post. It would not be believable if the reports were always 100% positive. From my experience and from observing the solo travelers on the tours I have taken, most people enjoy themselves and find they were worried about nothing. You will never know how many solos might be on any trip, so I suggest you pick an itinerary that interests you and go with it.
Also...plan to arrive in Rome at least a day or 2 ahead of time. Stay at the tour hotel so you don't have to move. They will have maps at the front desk and they will give you restaurant suggestions. Take a taxi (or in Rome a shuttle) from the airport if you are anxious about finding the hotel or managing on public transport. I just put that in my budget because I don't like to cope with public transportation when jet lagged. You may want to stay a day or 2 extra in Florence. There is so much to see there!
I also have experience with traveling solo on 4 Road Scholar tours, 2 international and 2 US based. They were fun and had more solo women but I found the Rick Steves groups more active and the people more mixable, if that is a term.
Have we convinced you yet?