Not sure if thos goes in lamps.. but here goes:

How do Mercury vapor lamps work in Metal halide ballasts, but metal halide dosent work well in Mercury vapor ballasts of the same wattage? hmm, why is it that way? cause if i get a 400w Mercury vapor lamp it will work in my metal halide ballast right? cool. but i just wanna know why it works that way.

The metal halide lamps need a slightly higher voltage to start and operate compared to the mercury.

The start is the reason, why probe start MH need higher OCV. It is possible to overcome it by using low voltage (<1000V pulse) ignitor. Such ignitor you might made by connecting 240V/150W (or two 120V/75..100W) incandescents in series with fluorescent starter rated for 40..80W lamps (i don't know the type marking of these in US, somebody might help you here) and connect this string in parallel with the lamp.
You should never use HV ignitors (>1.5kV; those for Pulse-MH or HPS), as they might trigger an arc in the outer bulb when attempting hot restrike, even if they appear (obviously, with the higher voltage) as more reliable - they might easily kill the lamp.