Are we allowed to have a 6 volt circuit on our robot?
It depends on what you are trying to do with it. If it falls into the “custom circuit” category (i.e. lights, camera, on-board vision processing board); then yes, you can have a 6V circuit, as long as it is powered from the PDB or it is a COTS computing device/camera with a built-in battery. There is also a 6V supply available on the digital sidecar to drive servos. We’re going to need a little more information to determine if your particular application is legal, though.
I don’t see anything that specifically says that you can’t create you own 6V rail on the robot! However, be careful!
Custom circuits can contain their own power supply within limits for custom circuits. If you have a COTS device that you are using i.e. a single board computer that requires that supply voltage, and it does not control any motors, solenoids or relays on the robot, then you are good to go.