If I already have a Roma Pass, do I have to make a reservation? Can I "skip the line" without a reservation for a tour?
Yes but you can't skip the security-check queues: no one can skip those. Depending on how busy the Colosseum is on the day that you'll be going and how many people are using Roma passes, that could involve a little bit of a wait.
No need for a reservation.
Find the entrance for Roma Pass holders. You'll go through security, then you're off.
May want to download Rick Steves self-guided tour podcast or, tag-a-long with a tour group, lingering in the back.