Can one use high wattage fluorescent tube lamps with low wattage ballasts?

It sounds fine. Think of the situation in this manner. If you have a low watt buld and a high watt ballast, your bulb will burn with the required brightness, but will burn out faster due to the higer watt ballast. If you switch the two and go with a low watt ballast, and a high watt bulb, you will still produce light from your high watt bulb, it just may not be as bright as its potential but it will last longer and not burn out as quickly. This is how it was explained to me. You say the lighting is fine, i say you are fine