Joining songs with different volume levels

It's unclear to me whether you need to edit the individual tracks of the projects when compiling the medley.
If not, you need to first export stereo wav-files of the songs, and then import them in one project to build the medley. 
 
When you export  you can already mix the songs to somewhat same level. Route everything through Master Bus and see that in Master Bus the RMS (=average) level of each song is about the same (around -10, not louder than -6) and that nothing goes over, say, -0.3 dB. Use a limiter or a compressor with a limiter for that. This does not make the perceived loudness exactly the same, because the loudness also depends on the frequency content of the tracks. This means you may need to use volume,  EQ and/or compression automation in order to adjust the perceived loudness of the songs.
 
When you have all the songs as one stereo track in one project, you can start by Normalizing the track to something like -1.0 dB, maybe even -0.3 . However, if there's even one peak in any of the songs that already is that loud (limited during export), the Normalization has no effect.
 
If you must edit the individual tracks to create passages or such when doing the medley, you must import (copy paste) all the projects in one project after another, and use volume, EQ and/or compression automation in Master Bus and maybe on individual tracks as well to even the level changes.