My group made the transition to in ears a couple of years ago, and have had some struggles with the transition that perhaps you could learn from. We went back and forth (and back and forth...and back and forth) about the whole "the only way we can blend is to hear exactly what the house is hearing" debate. I think that what you need to remember is that a monitor mix is a tool to provide your best shot at tuning and balancing the vocals. Let me see if I can clearly state my rational for why different monitor mixes for the band are OK.
1) The monitor mix will always sound different from the house mix anyway. There are many reasons for this...the frequency response of the monitors is vastly different than that of the house. Also, the acoustic space is different. The house is big and reverberant and will accent certain frequencies over others. If you're wearing in-ears your head is the acoustical space. Even different places in the house will sound different. It's a little crazy to assume you can know what things sound like out there.
2) Bone conduction in your head screws up how somebody hears a monitor mix. It takes a certain amount of "more me" syndrome to get enough of yourself in your ears to overcome that "I'm singing with my ears plugged up" feeling (which totally sucks). If I had to be subjected to everybody's "more me" requests, everybody else will be too loud and I'll have to turn the beltpack down, which will cause me to have to turn myself up to get over that ear plug feeling.
3) Consistent mixes will take care of the balance issue because of relative balance. Lets say there's 4 people in your group. Person 1 and 2 tend to sing at an average volume. Person 3 tends to sing a little quiter. Person 3 tends to sing a little louder. The house sliders or house gain will probably be set to account for those differences. All you need is a consistent (unchanging) mix to balance to each other in this case. If person 2 starts to sing a little louder, everyone else will hear this change and adjust accordingly. If person 4 starts singing a little softer, everyone can follow along. Because everyone's relative level is set at the board, and what they hear in the ears doesn't change expect for when someone starts singing louder or softer you're probably OK.
What you don't want to do is have people listening to a house mix that will change as the engineer adjusts levels. If singer 3 (the one who tends to sing quiet) is listening to the house mix, and the monitor engineer decides he needs to be bumped up a little, singer 3 will hear that change as him being too loud, and start singing even softer...which will make the engineer turn him up louder, and now you've got an awful game of "level tag" happening. This reason alone is reason enough to not listen to a house mix that will change.
And now the biggest piece of advice: Stereo Monitor Mixes
We spent the last two years trying to work with one mono mix for each person. We could each control our own mix, but only got it in mono. We couldn't change the panning of where other voices were in our head. This was frustrating because it was hard to distinguish my voice from everyone else's. It became a big mush of noise in the middle of my head. Not a very nice thing to try to use for tuning and balance. So, we all ended up taking out one ear or even both ears trying to get something that didn't sound so unnatural. Your head is used to listening to things in stereo. Why deny it that luxury?
We've recently changed over to where we are sharing stereo mixes. We used the same board and paired up our mixes, so instead of six mono mixes, we have 3 stereo mixes (plus one mono mix for our VP who still uses a wedge). 2 background parts share a stereo mix and pan their two voices close to center, the other 2 backgrounds farther out, and the bass and drums to the farthest left and right. The bass gets his own mix because no one else will probably want to hear what he hears. We're able to listen to a more natural mix now, and we're able to keep our monitors in both ears.
If you use stereo mixes we've found stereo ambient mics helpful also. These are two mics (one on either side of the stage) that only feed into the monitor mixes (not the house). Pan the stage right mic all the way right, and the stage left mic all the way left, and what you get is a nice "real" sound that helps make the monitors feel a little more natural. Plus you can hear all those crazy things the crowd shouts at you. They're a nice easy touch to make the mixes more comfortable.
Two manufacturers that I know of manufacture a small format mixer specifically designed to provide stereo mixes for in-ears. Allen and Heath Mix Wizard Monitor is one, and Crest XRM is another. Both allow each singer to control the volume and pan of each signal independent of all the other signals. They are super cool.
Sorry for the novel. Hopefully that was somewhat helpful.
Mike Henrickson
Cartoon Johnny
www.cartoonjohnny.com