A:
Its not so much that doing so isn't allowed - its just that no good comes from doing so when the potentials don't match.
For equations:
1) try picking a wire size and length, or a trace size and length - calculate resistance therefrom
2) Using internal battery resitances and the conductor resistance, calculate current flow and the temperature rise per unit time - compare to heat dissapation rate
3) impress the interviewer with your ability to gage not only the time required for the copper to melt, but the ammount of light (if any) produced
4) further impress the interviewer by calculating battery temperatures after the wire pops - also calculate amp-hours remaining in each battery