Ballast

Do I need a Ballast for all wattages of HPS or Hay lights? I have a 400 watt HPS that I run off a ballast, If I was to buy a 100 watt Metal Halide would I need another ballast or would the light fixture be sufficent.

I want to get another bulb, does it have to match my ballast ?

Yes Ballasts and Bulbs Should be Matched.

HID bulbs generally need specific ballasts, and any given ballast can usually safely and effectively operate only one type or a few types of HID bulbs.

The bulb wattage must be matched to the ballast. A smaller bulb will usually be fed a wattage close to what the proper bulb takes, and will generally overheat and may catastrophically fail. Any catastrophic failures may not necessarily happen quickly. A larger bulb will be underpowered, and will operate at reduced efficiency and may have a shortened lifetime. The ballast may also overheat from prolonged operation with an oversized bulb that fails to warm.

the socket you need for the bulbe should be the same, E40 type socket, what you would need is a different ballast.

but why would you go with such a small MH? in this size the are as effective as a mid sized CFLs. so really, don't start below 400W with MH/HPS or you wasting money away