If you have a 1000 watt ballast you can reduce the size of the bulb only. Do not use a dimmer you wil have problems and maybe fire.

So I'm running a 1000 watt hps and a 400 watt mh conversion bulb in a 1000 watt hps balast. I also run a 250 hps bulb in a 1000 watt hps ballast. Do not mix hps and mh they wil burn out. Unless it's switchable ballast or conversion bulb.

So bottom line is Equal to or smaller bulb in a balast. NEVER a larger bulb than the ballast. It will explode.