I have a 175w mercury ballast I want to add a cap to for better lamp power regulation. 

What value do I need for the cap? 

I believe the cap is hooked up between the lamp lead from the ballast and the lamp socket in series with the lamp right?

Finally, is there a chance this might not work with certain ballasts?

nope...it WILL work....I have a HX ballast......and on it says optional capitor....the reason why he is asking is because I told him I had a HX ballast that said to make it HPF you can add an optional ballast!

Okay Dave checked his M250R for me.  It's wired the same as my NEMA ballast but has a cap between ballast output and lamp center contact.  Cap is 13.5uf I believe...

Some low wattage metal halide fixtures use HX ballasts equipped with capacitors. So you'll be fine.

I've tried adding caps to a couple different hx-npf ballasts and for some reason the lamp has trouble warming up.

If I short the cap...same as removing it, lamp will warm up no problem...then put cap back in and lamp will stay bright. Line current will go from 3 amps or so down to 1.7.

I measured the starting current in lamp with cap to be about 2.2 amp...almost the same as with none. I warmed up a normal cwa lamp to full brightness 2 times while I did this experiment and the lamp in question was still only half bright. I'm puzzled as to why this is.

The most recent ballast I tried this on was from a cheapo 24 dollar security light from Lowes and the cap was 15 ufds.

@SeanB: Yes, I forgot to mention the line caps. In the online ballast specs for Advance and a few other manufactures, they list a line cap as "optional" for reacter and HX-NPF ballasts on many of the circuits. I've seen some as high as 35 uf on the smaller wattage lamps. The starting current is still a little high...on my 175 watt NEMA head I used a 50 uf cap and lowered starting current from 5.5 amp down to 3.5 amp. And full run up current from 3 amp down to 1.8 amp. 

Whether cap is series with lamp or parallel with supply, make sure cap voltage rating is higher than voltage being used in the application.

I just need to find a cap now...  Preferably one of those round plastic kind, it's going into my NEMA head with Advance ballast.

I once mounted a plastic cap into a NEMA head and I think it gets a little too hot in there...after 100 hrs. or so the cap had cracks on it.

That sounds about right it does get very hot in there

I'm going to do some tests on a newer Cooper NEMA ballast with the cap in series with the lamp and parallel with the power to the ballast and see what happens.  I will also test on my 1986 NEMA with Advance ballast...

On my older ITT 175w NEMA head, I have the above mentioned 50uf cap remote mounted now...but that fixture is indoors. If I was mounting one outdoors with a cap, I would probably use an outdoor weatherproof box. It would connect to the white and red wire from photocell...if the cap stayed energized, especially big as 50uf, it would draw about 2 amps reactive power. Just a thought.