I have recently written/recorded a song and mastered it. It was quite an extensive production using midi and recorded guitar, alot of tracks were used. Infact, it was a little to much content for my computer to handle so when i tried to play back it ran slow =[. I was able to master it fine though and now have a nice master sounding instrumental track.

now the question is this: if i want to add vocals or maybe an idea i had to add to it,

what would the difference be between adding the vocals into the first session or starting a new session and importing the instrumental? if i remaster this session will it idk...over master the instrumental?

im using Ableton live btw, dont know if that matters.
im kind of talking about both, like is it better to mix vocals into the first mix or is it safe to just mix one instrumental track with the vocals, then master it again.

im worried about messing up the quality
don't master until the mix is finalized, also you're supposed to master the mixed down audio, which means it's one stereo track, don't master it before it's been mixed down.
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It sounds like your confusing mixing down and mastering......they're two different things.