If you must get a smaller light all you need is the bulb. As long as the ballast is bigger than the bulb it will run nice.

My Bro runs a conversion 400MH for veg and a 600HPS to start 12/12 then a 1000HPS a few weeks later to finish. All on a 1000 watt HPS balast. I know one guy who got in a pinch from a blown light and got a 250HPS from slows. Saved his day.

Heat is what most have to over come. Hope you get it.

Remember the bigger the light the bigger the bud.