Equivalent voltage rating of Parallel connection of capacitors with different voltage rating?
You cannot apply a voltage higher than the lowest rated capacitor voltage to the string. If you have 3 capacitors, one rated for 16V, one rated for 35V, and one rated for 100V, you cannot apply more than 16V to the string. Realistically you should even derate the minimum voltage. I would not apply more than 12 volts to a 16 volt capacitor.

Generally speaking, when capacitors of equal voltage ratings are placed in series the voltages are added together. That being said, if the capacitors have different voltage/capacitance ratings then there is no good way to determine the overall voltage rating. The "weak point", so to speak, could be anywhere in the string depending on the order of the capacitors. (Thanks Neil for pointing this out)