With regard to the Voltage ratings of capacitors, it is almost never a good idea to use one with a lower rating. This is because it is usually safe to assume that the designer chose the Voltage rating specifically to perform at the Voltage in the circuit, less whatever margin of safety the designer feels comfortable with, then chose the capacitor with a Voltage rating nearest to that which was commercially available, without going under his design's lower limit. Practically speaking, you can usually choose a cap with a higher Voltage rating, although you may not find the increase in size convenient.
As far as the capacitance rating is concerned, what you can get away with will depend on how the capacitor is being used in the circuit. If it is an output cap, for instance, the capacitance value may very well determine the lower limit of frequency it will pass. If it is a power supply filter, the capacitance rating can effect other things.
Perhaps the designer will stop by and answer you specific questions.
