How to power a Raspberry Pi with a flight controller
The flight controller has onboard 5V (and 3V3) regulators, so you probably might power it directly via the LiPo pack. That said, the raspberry draws at most 700mA, your board and your camera and no servos (is there a gimbal maybe?) would not exceed your four-in-one regulator BEC maximum current, that is 3A:
  

You might have problems only if the Pi is particularly susceptible to the disturbances coming from the switching regulator (I don't think so) and from the motors (that might be).
Just try to use only the BEC you have, your Pi will probably be good.