Can I feed lower than rated voltage?
It depends on the component what a rating means. Some voltage ratings are such that you should supply the device only the rated voltage (+/- some tolerance). Examples of these are most IC's/complete devices, which won't work properly when given too little voltage, and can be damaged if given too much voltage (not to say that too low voltage can't damage the device, or too high a voltage will allow a device to work properly).

Others are a simple max bound. Any voltage below the rated voltage is fine. This is the category capacitors fall into. The higher the limit, the better. So both the 50V and 16V capacitors will work just fine with a 1.5V battery. However, only the 50V capacitor will still work if you decide to hook up a 24V battery.