[pkautzsch]

What you might try, if you want the piano close to the choir (I suppose it's for rhythm precision reasons?):
Have the piano between omni pair and choir, lid opening towards the choir so its reflection doesn't hit the omni pair directly. The omni pair will by this become more of a room pair than a main pair and might be spaced rather wide - like 3 to 6 ft maybe.
Then, add four spots for the choir - cardioids or hypers probably - and a piano spot. People often suggest ribbons for piano here on gearslutz, lacking a ribbon mic I've never tried that though, so I won't comment on that. Neumann KM1xx can make the sound very bright though. For choir, I prefer Schoeps most times. Maybe 414s for women and 140s for men? Would be worth an experiment, if there is enough time for such a complicated setup.

The best approach would be, though, to have the piano balance to the choir, and doing it Plush-wise.