I don't think you'll find one. The only real requirement is that they all be electrically connected. So, you can tie them all together, or not tie them together, and it's all the same since they all tie back to the EGC.
Now... you *may* run into issues mixing high and low current circuits and ground paths- if you have a 20A and a 200A circuit going into a single metal box, the metal box is required to be grounded, and I'd argue that any metal in the box- including the bare #12 conductor- is "likely to become energized" and would be required to withstand the fault current from the 200A circuit ...which would require bonding the #12 and #6 ground conductors. So, in that case, 250.122 would apply. Although it *still* doesn't require them to be bonded, it's just the practical thing to do.
Might also come into play if you're mixing #12 and #14 in a box.