I hope it isn't bothering anybody that I am asking so many questions lately, but I am curious about this one. I was wondering if I can use my 400w MH CWA ballast for a 175w MH bulb and possibly a 175w MV bulb. The ballast currently has a 24 uF capacitor, but I have the capability to go from 0 to 25 uF in 2.5 uF increments, so I would start at 2.5 uF, measure the wattage, then go up to 5 uF if it isn't sufficient and so on until I get the proper wattage through the 175w lamp. I have the following questions:

1: I doubt this would harm the ballast, but if any of you think it will, please explain.

2: This might be a bad question, but MH ballasts can be used on MV bulbs, right?
Thank you for your help! 

Yes, reducing CWA ballast capacitor was used in US to dim street lights (though to less extent than you propose), BUT by reducing ballast bapacitor you are going more and more into pure capacitive ballast territory, where capacitance dominates ballast impedance. And it was learned many years ago, probably 100+ already) that discharge lamp do not work well on capacitive ballast on line frequency, because combination of leading impedance and lamp's negative resistance is causing current spikes and oscillations. And so high current crest is killing the electrodes. Already somewhat noticeable on stock CWAs. So better don't, just get a 175W ballast, they are ubiquitous.

AFAIK, American 175W mercury and MH lamps are homologated by running current and voltage, so mercury lamps can be run on MH ballast. There may be some nuances with OCV (not much relevant to merc lamp) and use of ignitor.

Thank you so much for letting me know! I didn't know that the type of impedance mattered.

There is also another problem with "reducing CWA power by changing the capacitor":
Normally these ballasts are designed with the inductive component in series with the capacitor to be current dependent (shunt saturates, so reduces the inductance when the current goes higher) and by that detune the series LC so the current stays where designed. This current is dictated by the point on which current the shunt saturates. This forms very strong current stabilization mechanism, which suppresses very broad mains voltage variation, as well as to big extent even the capacitor tolerance.
If you just reduce the capacitor, you decrease the lamp power, but the shunt won't be saturating anymore, so the current stabilization effect will be gone.
Does not have to be that much of an issue when the mains voltage and all components stay within tight tolerance or when it is used just to dim a higher power lamp where the tolerance for the exact operating power for that settings is very wide.

interesting
I have a GE M400 250/400 watt HPS and to change between 250 AND 400 you unplug a lead to a section of the capacitor and I assume taking part of the choke coil out with it
and wondered if in theory having one set up say 150-100 and a relay to switch after midnight - I have seen 2 ballasts for one bulb fixtures and they switch between to run dimmed from midnight to around 6 am when it is reverted back to "high power" and if something like that was ever built?