You can use a USM 69 as your main stereo pair, and it is definitely capable of M/S pickup. But that is a microphone with a definite "sonic signature"--it doesn't have a smooth, neutral treble range. Thus you might need to keep it back a little farther away from the sound sources than you might initially expect.
Doing that can be very helpful anyway when recording in M/S or Blumlein, since it helps you to fit everyone within the coverage angle of the stereo pair. In M/S recording you must never let the side (S) microphone pick up any source of direct sound more than the center (M) mike is picking it up--the musical balance as picked up by the M microphone alone needs to be complete.
--best regards