I had to do this once for a computation CD for a charity event. I think 18 songs all from different performers, some mastered some not some recorded at home some at studios and a live track. Also various formats, some came on an audio CD so they were 16bit, some came as Wav 24 bit some 16 bit, one Aiff and one came as a file that we could not even open, lucky a mac laptop in the studio opened it and converted it to wav (still don't know what it was?).
The mastered songs I pretty much left alone, if they had been mastered very hot (some actually peaked the meters) I dropped the volume to -0.3dB. The songs that had been mastered a bit soft I brought up a bit to match the louder ones. And the ones that were not mastered were EQed and fiddled to fit in with the mastered songs.
When I had the album playing so that the track volumes worked, I.E ballads were not louder than rock tracks, I then inserted a very mild limiter across the whole thing but had it set so that input was - 0.4 and the output at -0.3db, that made the limiter just grab any wayward peaks and actually just seemed to make the tracks sound even more like it was an album.
The album was well received and a few of the artists commented on how good their songs sounded on the CD.
Alan.