Rick suggests that the Colosseum might be skippable (the best bit being the outside), but I'd like to have a peek inside as well. One option is to schedule a night tour. A few years ago, a poster here had a very negative experience that seemed to be centered on a required video at the beginning of the tour. Not sure if they still do that years later or if anyone has an updated report on the experience.
Right now, I'm leaning toward just having a combo tour during the day of the Palatine Hill, the Forum, and the Colosseum since I'll want to do the others anyway.