Usually not. You may find an exception on the net somewhere, but usually, a ballast rated for one wattage only uses a bulb of that same wattage. What you might do is get the 1000 and veg for a shorter time. I'd ALWAYS opt for the bigger light, myself, unless the space is so small heat becomes a problem.

Are you planning on the 600 because they're more efficient? If so, the difference isn't THAT great if your only running one light. Maybe stinky will pop back in here to comment on the shorter veg time with the 1000 idea. I'm learning the finer points of lighting myself---so I'm certainly no expert.