Rob, my 2cs worth ...
At what point do people move onto the next stage? I know for stage 1 Justin says to get at least 40 changes a minute for the one minute changes and to be able to play one or two of the songs even if you have to start and stop a little bit.
I think you can make your decision to move to stage 2 based on your One-Minute-Changes and Strum-Pick-Strum exercises. Once you reach 40 on OMC and are generally clean and comfortable on SPS then I'd say move on.
Many folk continue to work on songs using A D E as they start learning Am Dm Em. I think that is fine.
Less fine is starting and stopping ... a widely accepted principle is that whatever you do in practise is what you will get down. So best is to practice being smooth, without hesitating, however, slow the tempo to allow that is. As you get it down then increase tempo and brain is better able to make this happen, since it is "just" speeding up what you can do.
That is assuming the hesitation is caused by not being able to change fast enough rather than being uncertain of what chord comes next. If that is the case then I'd suggest writing the progression down to aid in learning it and committing it to memory.
At what point do people move onto the next stage? I know for stage 1 Justin says to get at least 40 changes a minute for the one minute changes and to be able to play one or two of the songs even if you have to start and stop a little bit.
I'm wondering more with regards to the tempo of the songs. For example, Hound Dog is supposed to be played at 175bpm (according to the internet) but that seems really fast for a stage 1 guitarist.
So, at what point did people decide to move on from a song? Did you work to a percentage of the tempo e.g. 70% and keep revisiting it over time alongside newer stuff until you could play it at the correct tempo? If so at what stage did you stop doing that and were able to play at the full tempos?
Don't worry about the original tempo as a beginner. Being able to play it smoothly on time far more important. Over time you will speed up. And you'll find, even playing a song much slower than the original, will still sound good if you are playing steadily, without hesitation. Listen to how good songs sound when Justin walks through the chord progression playing just 4 simple down-strums really slowly.
Also, should you be able to play it without the metronome before moving on?
No, I think in the early stages stay with the metronome. I did lots of work with the metronome and probably stopped practising my strumming patterns with the metronome too soon. Now when I record I either use a metronome/drum and try to be solid or play and sing together and wait for feedback as to how my tempo is drifting faster/slower and my rhythm is steady.