Read in Rick's guidebook about using a private guide for the Colosseum, Forum and Palatine Hill. He said they stand outside the Colosseum looking for business. Has anyone done this? Thanks!
Regarding hiring a guide outside of tourist site – it depends on your standards. I tend to do a fair amount of studying before traveling, and it makes me crazy to be dealing with a guide who's making things up or knows less than I do. Consequently, I've never taken the approach of hiring someone on the fly. On the other hand, if you prefer to travel without prior preparation, it may work fine for you.
In Rome, I now use Context Rome exclusively. They limit groups to six, and they have guides who are engaging and are guiding in their area of professional expertise. Almost all have relevant graduate degrees. Also, the very small groups really makes it easier to move around a crowded site. You might take a look at their website to see if their offerings interest you.
The Colosseum has a 5E tour that lasts 45 min. http://www.coopculture.it/il-colosseo.cfm#
See site for times presented in English.
This might also work for you. http://www.romefreewalkingtour.com/free-tours/colosseum-tour/
Have not yet used, but they get good reviews on Tripadvisor.