Is it okay to run a mercury vapor lamp of lower wattage (100/150/175W) on a 400W MHL/MVL ballast? Also can a 60Hz ballast be used on 50Hz?
No.

A halide ballast must do 4 things well. So consider existing ballast specs as very important.  MH bulbs are still the best white light sources for even spectrum.


Providing the correct starting current.   
Providing the correct starting voltage.   
Providing the correct sustain current.   
Provide the correct restart profile to a hot bulb after a power outage. 


It may also be possible that the capacitor is the only thing worn out. which might be $15 similar to large microwave caps. 26uF 330Vac 0.82MOhm which can be tested by some techs.

https://www.amazon.com/RAB-Lighting-BMHH400PSQ-Halide-Ballast/dp/B00415KW80