We visited the Forum and Palatine Hill on a separate day from the Colosseum, and followed the tour printed in Rick Steves’ book for the first two locations. For the Colosseum, though, we booked the Underground tour guide upon arrival at the Colosseum ticket window (this was December 2012 - not sure if the time of year made a difference or if tour booking tactics are different now, 7 years later), but seeing the Colosseum from below the arena floor, with its passages and drainage features up close, and looking up into the seating area (and standing near the Mussolini-era seat restoration attempt section) all were well worth the extra time and cost. We even got an elevator ride as part of the underground tour, which saved a bit of overall energy. They waited another 15 minutes after we got our tour booked for others to sign up as well, so we hung around the entrance for a bit before our tour took off. We got to stand on the platform at ground level, too, while other visitors were confined to the walking in the seating areas and passageways to the outside of those upper seating sections.
So I can’t give direct suggestions for guided touring of the Fourm and Palatine Hill, but can recommend that you do opt for the Underground, as well as the rest of the Colosseum!