Obviously this is not a commercial CD release, meaning you will not have hours after hours of paintstakingly modifying mic positions and moving performers around.
My first try for this kind of setup is generally ORTF. It will work decently in most acoustical rooms and will catch a good representation of the actual performances if you have a good pair of cardioids. Unfortunately, this setup puts rather heavy demands on the "off-center" sound of the cardioids, some of the low-priced alternative are not so good there. I guess you can look it up, but 17cm and 110 degrees are the measurements, not necessarily to be followed religiously. You need to test at home mounting your mics on a stereo bar atop the tripod as there are some surprises to be had, especially with small diameter cardioids (hint: get a couple off distances to put under one of the mics). In fact, to me it has become such a standard that I have invested in a Schoeps MSTC64 mic setup, a stereo mic forever fixed at ORTF setup. Running that into a Sound Devices 722 with a Manfrotto stand gives a quite portable setup with respectable sound quality.
Try to get the mics high up, preferrably at least 2 meters above the stage floor. Exact placement is very dependant on room and ensemble so you should listen real carefully in closed back headphones to be certain. The full ensemble should fit "inside" the mics, ie the appearant angle of the performers should be less than the 110 degrees of the mics. This probalby puts the tripod out in the audience more than on stage but it all depends. You can modify the balance between solo / piano by moving the mics forward backward while listening on headhones. Closer will give more appearant volume to the solo that is generally placed in front of the piano.
My guess is that you can place the mics rather close for the string quartet and then back-off a little for the combined setup, but again, listening is the only clue you will have.
Best of luck, it is a bit hit-and-miss.
Gunnar
Gunnar