Regular metal halide bulbs will work in an HPS ballast setup - It will shorten the lifetime of them though. A MH ballast and capacitor, and an HPS ballast and capacitor are the same thing - the only difference is HPS has an igniter and the ballast usually has an extra line for this igniter.

HPS bulbs are under, well, high pressure, therefore need the additional voltage to arc and ignite. The MH does not need this extra power, but its still there on startup cause of the igniter - so an HPS ballast setup is much harder on MH bulbs.

It will work, but for bulb lifetime, get a conversion bulb - otherwise you'll cook em regularly