Warning - no real mix here, I'm just arranging things and this is a tiny clip from the end of the first song on my disc, which will, point of fact, be out before Drew's. Pardon the sloppy playing - it's all just throw away.
I have a few things going on here, and the mix gets really crowded when the piano comes in.
Soundcloud is being a pain in the ass right now, but this should work eventually:
There's a lot going on here, and I'm not 100% sure where to start mix-wise when blending in the piano part. The rhythm is quad-tracked, and there's the main "low" lead, a high lead, the piano part, and I actually have one more layer on top of it that I haven't done yet that's one more harmony that plays a kind of syncopated counterpoint to the piano.
Where should I be thinking on putting the piano in, mix wise? My original thought is to leave the low lead center, pan the two high harmonies left/right some degree (maybe like 75%) and then dead center the piano, but I think the piano laying right on top of the low lead might squash it.
If I have two lead parts that I'd like to be more or less central but don't want to interfere with each other too much, I tend to go for a 5-10% L and R spread. Subtle enough that they still sound fairly central, but not so subtle as for them to tread on each other's toes.
In all honesty, though, it sounds like your main problem at the moment lies with getting each individual element of the mix to sit in their various EQ pockets - everything sounds extremely muffled as it is. When you've got quite as much going on as there is here, you can get pretty brutal with hi and lo passes to try and get a bit more separation. Having said that, the rhythms actually sound like they're nothing but mids at the moment - not meant as a criticism, but you're always going to struggle to gain clarity if everything is fighting it out within the same bandwidth of frequencies.
Hope that helps/makes any sense!
EDIT: D'oh, forgot to say - with harmony parts, 75% L and R might be a little excessive. I'd normally go for a little bit more overlap to emphasise the interplay between the harmonies, but I'm no expert and I'm sure plenty would care to disagree, haha. Still, the only way you'll really know what works with your mix is by playing around with various degrees of separation.