What exactly do you mean by complexed mix? Can you give an example?

Mainly the vocal crossfading. Multiple vocal harmonies going in & out. Maybe a tune w/ variable time signatures. Things of that nature.

organization... if you're using a DAW then that might mean making markers at each "change" of the song, making sure every track is named efficiently, sorting the tracks in order of importance, ect.. that sorta stuff. If you're using Logic Audio, then they have these things called 'folders' which can be very useful when managing tons of tracks at once (it's a sort of hierarchy thing). Also there is routing to consider. Many times complex drums parts will be organized so that each tom will have it’s own fader, and those will be bused out to another fader named “toms” and that fader will be bused down to a fader named “drums”. So the same could happen with vocals and back-ground vocals, and groups of similar instruments. Things like changing time signatures and tempo, I’ve just found Logic to be just fine. I’m sure any program can do it with the same amount of success. I’m not sure exactly what you are asking. Just get in there and if you need to, keep notes on what’s happening in the song. Get that pad of paper out!

Obviously if you are editing lots of small segments there is probably going to be some sorta of crossfading action! Some DAW programs handle this differently, as some will actually render out a small audio file that is the actual crossfade, and store it in a folder named “fades” or something like that. They probably do this to save the cpu time it may take to crossfade on-the-fly. I personally can’t stand it though. If I’m doing some crazy editing with tons of crossfades, I use a Sonic Foundry Vegas, as I’ve found it to be much easier, faster and more accurate then what I’ve tried doing in Protocols or Logic or Cubase.

I’m not sure if this addresses your question, but when mixing, and having tons of instruments going on all at once, it become very difficult to hear everything clearly. So when dealing with EQing, I try to follow the saying “The more instruments you are using, the thinner each one should be. The fewer instruments, the bigger.” So in this case, each instrument may sound like crap when soloed, but in the big picture, they really can’t be taking up too much room because, obviously, it usually starts to get sounding like a jungle in there. For big vocal harmonies, you may need to do frequency juggling so they won’t fight each other at the same frequency, or simply just turn one of them lower or pan them differently. This is also where that routing things I mentioned first comes in handy, you could have a fader “backup vocals” and eq that and it will apply to all the individual bg vocal tracks that are being bused to this fader.

I hope that addressed your question. Those are just some ways I approach complex mixes, I doubt every one goes about it the same way...

I have no clue as to why changing time signatures would have any effect on a mix whatsoever. Maybe you're over-thinking?