A friend and I went to Rome a couple of years ago, strictly on a tight budget, of course, and we discovered an amazing thing. American students, studying in Rome, give guided tours during the summer, from May onwards, around some of the larger tourist attractions for FREE! They do this in the hope of attracting you on paying tours, which, while i'm sure they were worth it, we still couldn't afford! The tours are very informative, including ones around the vatican and the forum, which to the untrained eye looks like a field with lots of rocks in it!
the students have no problem with you not joining the paying tour, there is no pressure and they're very friendly and will probably be able to answer any question you have.
They won't look any different from other tour guides, but if you wander around looking bemused, they usually find you!
Have fun!
Gylly.