The private guides I have worked with for tours in Vatican City are:
Patrizia Sfligiotti ([email protected]): I toured St Peter's Basilica and Castel Sant'Angelo with Patrizia last year. Learned so much! Patrizia is a former archaeologist who has been involved in several of the major digs in Rome, including the site that ultimately became the Crypta Balbi Museum as well as the Tomb of the Scipios on the Appian Way. Patrizia studied at the Vatican as part of her higher education, and she brings a wealth of knowledge to her tours (I did a tour with her in 2016 at the Tomb of the Scipios as well as Monte Testaccio - both are fascinating sites!).
Agnes Crawford: My first tour experience with Agnes was a tour of the Vatican Museums, Sistine Chapel, and (briefly) St Peter's Basilica as part of a Context Travel tour. This year, I booked a private tour with Agnes to explore Medieval Rome (you can read my TripAdvisor review of the experience here, and there are tons of reviews about Agnes' tours on Trip Advisor - look up Understanding Rome Tours). Agnes' breadth of knowledge blows me away, and I can't recommend her enough.
Regarding your distance question... When it comes to navigating Rome's historic city center, my experience has been to use Google Maps info as a frame of reference, but NOT as a hard, fast rule. If you take a taxi from St Peter's Basilica to the Colosseum area, that driver must navigate across the Centro Storico part of Rome, which has the densest amount of traffic and tourists. Depending on day of week, random street closures (VERY common in Rome), protests (yep, I've run into transit challenges due to protests as well), or any other garden variety hiccup, I'd say 20 minutes is low-balling the time to get between the 2 sites. Plan for more time for the transfer, and if you arrive early to your Colosseum-area destination for the bike tour, then you have time to grab a quick bite before biking.
Can you please clarify about these "skip the line" tickets for St Peter's Basilica? I'm unfamiliar with what you reference since St Peter's is free and no tickets are required to enter. You have to go through security, and there's no "skip the line" option for security. As others have so rightly commented, include a buffer of time for getting through security for your time estimates. Everyone on this forum has had different experiences with security at St Peter's... Sometimes it's a very short (if any) queue. Other times, the lines are insane. Always be prepared for the latter.
I defer to the others regarding the audio guide questions as I have no experience with either the RS or Vox Mundi audio guides.