are metal halide ballasts interchangeable?
 

i have to replace an icecap 250 watt ballast. Obviously I can't order a new ice cap. If I were to buy a different brand would i need new harnesses or plug ins to the remote fixtures? In other words, what else to I have to order besides just the ballast?

Most ballasts have proprietary plugs. Although I'm sure you could make it work somehow. Try asking the guys in the DIY forum.