Capacitors connected in series cannot be relied upon to share voltages well. Unless steps can be taken to equalise the leakage currents, it is likely that some of the capacitors will be more stressed than others. This makes the overall breakdown voltage unpredictable.
With such tiny capacitances, I am doubtful that you could get much performance out of the system by the time you had added leak resistors low enough to equalise the voltages.