Naturally, the left and right images of a stereo image must be of comparable quality for the resultant stereo image to be of satisfactory quality. This means that not only must the images be in focus, but factors such as the brightness, contrast, resolution and of lesser importance, aperture and photodetection system, must be preserved between the two images. As several of these factors are dependent on the environment of the images and the construction of the camera, the best way to ensure that these factors are kept as constant as possible is to use identical models of camera when taking the images (as in Section 2.2)7.
This is borne out by simply qualitatively observing images taken by an Indycam and an cam -- the images from the cam look much crisper, more balanced and much better overall. As such, mixing images from Indycams and cams would almost certainly result in poor quality stereo images.