I wouldn't be a puritan about this. If you rent a Neumann SDC omni with an sbk130 sphere attached, it's a reasonable substitute for the small diaphragm embedded in a sphere in the 150. It's not identical but as Ben says, "If it sounds good, do it"
The center mic won't predominate anyway and a stereo mic placed on the center leg can pull in the back row of a large ensemble when needed.
You may find that the m150s as a spaced pair, with a stereo pair deeper into the ensemble works better than the tree without spots or flankers.