I've wondered about compatibility between MUed units. The two high hood C420s that the Monon had for passenger service were used for freight service after the end of passenger trains on the Monon. Were the two high hood units geared differently than the freight units? And if so, how does that affect MU operation? On the southend, when a couple of C628s were MUed with a RS-2 in the middle, there was a signifcant difference between tractive effort and horsepower. In other words, I don't know how the MU connection compensates for locomotive power differences and gearing. There's a photograph of C420 #501 lashed up with RS-2 #56 and C628 #407 on page 104 of Lewnard's Monon book.
Obviously with the combination of various locomotives MUed together, engine differences made no difference to their operation together.
Stew