There is normally a very long security line for St. Peter's. The Basilica opens at 7 AM, and I have no idea how early you'd need to show up to be very near the front of the line so you actually get inside within a few minutes of 7 AM. I'm not sure how long one might spend at St. Peter's (haven't been there since around 1990), but it's very large, and there's quite a long walk (about 15 minutes) from Sr. Peter's to the Vatican Museums.
Bottom line: It really wouldn't work to plan to visit St. Peter's at 7 AM and the Vatican Museums at 8 AM unless one only wanted to pop into S. Peter's very briefly and was willing to arrive long before the official 7 AM opening time at St. Peter's.
For those who absolutely must fit St. Peter's, the Museums and the Colosseum into a single day, this might work:
Buy an early commercial tour (probably over €100 per person) that covers both the Museums and the Basilica. Those tours are usually allowed to use the back door between the Sistine Chapel (part of the Museums) and St. Peter's. They go to the Museums first, ending at tne Sistine Chapel, then whisk the group right into St. Peter's, avoiding the 15-minute walk around the outside of the building and the painfully slow security line.
- Buy an afternoon entry for the Colosseum.
Alternatively, one could buy an afternoon combination VM and St. Peter's tour run by a private company (the Museums might be more crowded then than for the early-morning tours) and a morning tour of the Colosseum.
The tricky part with either of those scheduling approaches is that getting tickets for the Colosseum is difficult even if you aren't trying for a specific time of day.
A disadvantage of depending on a combination VM + St. Peter's commercial tour to fit the Museums and the Basilica into a half day is that those tours are a lot more expensive than the general-entry tickets sold on the Vatican Museums website. (St. Peter's itself has no entry fee.) Only combination tours run by private companies are normally allowed to use the backdoor between the Sistine Chapel and St. Peter's, access to which is essential for seeing both those key parts of Vatican City in the morning.
On the positive side, the commercial tours of the VM + St. Peter's don't seem to sell out as early as the Vatican Museums' much less expensive online tickets (which don't get you through the back door into St. Peter's).