I tested the Mi 6 battery with the Pro1 phone and it ran off that battery fine. I don't remember if I tested charging the Mi 6 battery through the phone. At the time, I didn't have a working usb port. I have since received a working usb port. The phone is still disassembled, but I had to rob the screen/selfie ribbon cable for my Pro1X. Therefore, I won't be able to use the pro1 screen to see the current charge.
I could probably test charging the battery through the USB port by getting the current charge from my external charging board and then try to charge it from the phone connector