I was thinking of getting a different set of wattage mv lamps and was thinking of getting a 175w ballast because i think there the most common. (what is the most common mv lamp wattage?) I read somewhere that the arctubes of 250w lamps are the same length as a 175w arctube only thicker and because of that fact they can run on a 175w ballast a bit dim buts still fine. Is this true? Thanks in advance!

Even though they have the same arc length so same ARC voltage they are not compatible. The current is the important factor. By running 250W on 175W the lamp will be under-driven by a fair bit. It will work but you risk damaging the lamp and giving it an early grave by sputtering the electrodes away quicker than it should.
Lamp  V     A
250W  130V  2.13A
175V  130V  1.5A

250W has a longer arc tube than the 175W model, to maintain the arc loading at the ~60W/cm optimum (the exact figure depends on what the maker want to achieve, but it will remain the same over the complete lamp family). To maintain the same arc voltage across longer arc, it has just lower pressure fill (both buffer gas, as well as mercury).

Running 2.13A lamp on a 1.5A ballast means about 60%. It is not perfect, but OK for a MV. The life wont be affected, but the efficacy will. So it will emit less light than the proper 175W lamp (it will have lower arc loading).