And on no.1, the working voltage should ideally be half that or less of the caps rated value. Above that, the capacitance value drops off (so you might need to use a bigger value capacitor than normal if that's the case). And when working with AC circuits, if the cap only has a DC voltage value on it, then remember that the RMS voltage of an AC circuit is less than the peak voltage achieved by the AC signal. For a sine wave this is 41% greater than the RMS value.