When I look at the schematics, there’s no reason why you can’t mix and match driver modules with different communications approaches (ie UART vs SPI) on the Octopus Pro board. I don’t know if the same can be said for Klipper (but I suspect it can). I would not recommend mixing addressed UARTs with any other types of modules.
I haven’t worked with SERVO42C boards so I don’t know how the serial communications are wired. It looks like they don’t work well with Klipper:
I have no idea how you will wire CL42s into your system.
What is the “Nyquist phase shift”? I can’t find a definition for it.