My basic question: Is there a rule of thumb for choosing the keys for the components of a medley? I've never heard anything about this.
The first example that comes to my mind is from the Beatles. John wrote the "I read the news today / oh boy" bit that starts out 'A Day in the Life.' Paul met up with John and told John he had a song he had been working on, which starts out, "Woke up / got out of bed / dragged a comb across my head." They decided to weld the two together into one track for their album, effectively producing a medley of two songs. My songbook scores both of them in G, but just statistically speaking, it's likely that when the two fellows got together with their songs-in-progress, they would have been in different keys. So maybe this is suggesting that you should record both (or all in the case of more than two songs being joined together) in the same key.
Complicating my example above is the fact that a lot of Paul's notes are unnatural to G, but natural to E. I don't know how relevant that is to my question, though.
A point against matching keys would be that transitioning to a different key helps the listener identify where the different songs are welded together. But I don't have a feel for whether that is desirable, or whether one should keep things as seamless as possible by matching keys.
Maybe you know a rule of thumb for this, or maybe you can point me to some examples of medleys I can find at some website such as YouTube. I have some specific songs in mind that I might try to do as a medley, with just a verse and not much more taken from any given song.