I know I already asked this question about 250w PSMH, but I also have a fixture with a 400w PSMH ballast. And I was curious, can it run anything else?

If the ballast is a 400W M135/M155 pulse start metal halide ballast, it should run 400W H33 mercury vapor lamps just fine due to the identical arc voltage drop of 135V and operating current of 3.25A as long as you disconnect the ignitor to prevent starting probe and resistor destruction during hot restrike. In addition, you might also be able to run 400W M59 probe start metal halide lamps on 400W M135/M155 pulse start metal halide ballasts if the OCV is above 270V or so with the ignitor disconnected as well. In addition, you could also run the CWA compatible Iwasaki Sunlux Ultra Ace 360W high pressure sodium retrofit lamps and Philips 360W retrolux high pressure sodium retrofit lamps as well if you disconnect the ignitor as well since most 400W M135/M155 pulse start metal halide ballasts are CWA ballasts while there are a few 277V series choke 400W M135/M155 pulse start metal halide ballasts that could safely run standard high pressure sodium retrofit lamps designed for 400W H33 mercury vapor ballasts like Sylvania Unalux 360W lamps if you disconnect the ignitor on those ones as well, but you need a 277V supply to run the 277V series choke ballasts properly.

In addition, you can also run 175W H39 mercury vapor lamps on 175W M137/M152 pulse start metal halide ballasts as well with the ignitor disconnected and the same even applies to running 1000W H36 mercury vapor lamps on 1000W M141 pulse start metal halide ballasts with the ignitor disconnected as well.