Mate dont worry to much about harmonic mixing, Its like somebody else said it only works providing that everything is the same bpm I have mixed in key and it is handy when you get those tunes which just dont seem to gel with stuff, Its good for mixes that you would never even think of trying otherwise. I would never ever dream of mixing a whole set Harmonically it just sounds too perfect, I think this is kinda how Fresh's Sound Weapon works and if you listen to some of his sets from late last year everything flows well but I think in all honesty it makes it sound too perfect. Biggest problem though is the fact that doing it that way takes all the fun out of mixing, Some of the best mixes I do according to mixed in key shouldnt work, Coming up with mixes that sound so well together that you can't help but grin when you drop them in a club is the most rewarding thing about mixing. Mixed in Key is a great referencing tool but its not essential, Ive been mixing over 10 years and playing in clubs abt 8 and never once have I had anyone come up and say 'Good mix, however that one mix was out of key' So whatever this promoter is saying I would take with a pinch of salt. If its your first live set best piece of advise I would say is plan atleast your first 5 mixes cos generally if a set starts well you find everything else does, And if nerves get the better of you don't be afraid to have a couple of drinks first.
Talking from a promoters point of view the one thing I most like to hear is somebody who accepts you have to start somewhere and plays suitable music for that time of the night ie, If your a warm up dj play deeper, maybe harder but certainly no jump up, The way we run Bassix we like things to progress naturally as the club fills, maybe this night doesnt but I'd imagen you've been to this night your playing at before so try to fit in, Far too often we get kids who are new to playing out and so hungry that they completely ignore our music policy and think they're headlining, double dropping, playing jump up and completely forgetting what they're there to do, It is hard not too but from my point of view you would be more lightly to get another booking showing that understand and respect all of this rather than worrying about harmonic mixing. Good Luck!!