I think you can mix down to minor third from major, however, since your dropping down it may not sound exact, I think when your moving either to major or down from it, it's best to have some knowledge regarding whther or not the two tracks your mixing together will work well, as im sure in this situtation not all tunes work well dropping from major down to minor.
Futhermore, if you use the camelot wheel I posted in another thread, you can see two circles , the smaller "Minor" circle and the larger "Major" circle. from what Ive gathered you can mix from a minor song only into a major if the next song falls into the "Major" box directly above the minor song your mixing out (For example, 5a to 5b works, but 5a 6b does not because you are going up one and then one to the left)
I hope this answers your question.
