I used this one….very well organised and the guide was absolutely fabulous.I think the guy (Sean) put on a bespoke tour for us that fitted in with the bus service he was running for us to the game.
As @dazzer6666 said though - the structure of the route and all the information shared via the wireless earpieces was far beyond what I'd have learned on a general solo mooch about.
I'd drop them a line via their website and just say you're looking for a walking tour of the main sites.
Maybe do that one day - maybe the afternoon when you arrive - and then do your own thing on day 2 - selecting anything from day 1 you want a closer look at and/or do the paid entry for the Colosseum.

Rome in a Day Tour with Colosseum and Vatican Museums
See Rome in a hurry on our Rome in a Day Tour! Includes skip-the-line tours of Vatican Museums and Colosseum, plus Roman Forum, Trevi Fountain, and more!