In theory, since you will be configuring the GPSes yourself (not depending on ArduRover), you can set the baud rate between the Moving Base GPS and the Rover GPS to anything you wish. I had mine previously at 115,200 and it worked fine, but I was using exclusively F9Ps with 2 UARTS.
BUT, I do not think it matters that you can theoretically set the baud between the GPSes to what you wish because of the single UART issue posed by the C94-M8P:
The baud rate for any GPS UART connected to the Pixhawk in the GPS for Yaw configuration, needs to be 230400 as that is the baudrate that ArduRover will set up the Pixhawk GPS and GPS2 ports. When you set GPS_TYPE to 17 and GPS_TYPE2 to 18 (for GPS for Yaw), ArduRover ignores the SERIALx_BAUD parameters for the GPS and GPS2 ports and sets them to 230400.
For proper GPS for Yaw, the Moving Base GPS and the Rover GPS should be connected to the Pixhawk. Since you plan for one of those to be a C94-M8P, which only has one UART, that UART’s baudrate will have to be 230400. That, then sets the baudrate coming IN to that UART at 230400 also, which sets the baud rate of the GPS feeding it to 230400. I might be going around in circles, but it is clear to me, that you will have to have the baud rate at 230400 for all GPS UARTs.