We are booked for the trip in August, and looking to fill our free time on Day Two. We are trying to fit together seeing the Sistine and museum (via the Omnia 24h, since we missed the ticket window), Colosseum, and the surrounding Forum areas that are not included. We weren't sure if the tour included just the main Forum, or the Imperial and Palatine areas, too. We also wanted to ask if this even sounded feasible time-wise, or if it is more of a one or the other situation.
I think it would be virtually impossible to do both the Vatican Museums/Sistine Chapel with the Colosseum/Forum in the same afternoon.
Since you will be at St. Peter's the morning of day 2, it makes sense to stay at the Vatican and do the Museums and Sistine Chapel in the afternoon. However, please proceed with caution re the Omnia 24h card - what it promises seem a bit unrealistic.
https://www.omniavaticanrome.org/en/cards/omnia-card-24h
On day 3 it sounds like the tour will take you past the Imperial Forums to the Colosseum - where you walk around it, and then go into the Forum. Perhaps others who've taken the tour can comment on how much time is spent inside the Forum - it's a big place and can easily take 2 - 3 hours to go through it, if you include Palatine Hill.
One thing you might consider after the Vatican [and dinner] is to find the viewpoints of the Forum from behind the Capitoline Museums - they're up the staircase to the right of the Vittorio Emanuele. Then you could stroll down Via dei Fori Imperioli to view the Imperial Forums - being able to take your time, which may not be possible the next morning when you're on the tour schedule. The ruins are dramatically illuminated at night - so it would be a different view from what you'll see on the tour.
If you aren't able to get tickets to the Vatican Museums, you could instead purchase a ticket to enter the Imperial Forums and/or the Forum/Palatine for the afternoon of day 2.
And then of course if you're arriving a couple of days before the tour, you'd have opportunity to see more then.
Thanks for the thorough feedback. We ended up doing the Colosseum 24h with the Super Sites addon, and will probably do the Domus and Circus as well. Even if we re-tread some of the Forum the next day, it doesn't sound like we would be repeating a lot of that huge area. We also didn't want to pass up having a decent lunch in between starting at the Vatican with the tour, and making our way back to central Rome.