A couple of things first. Why are you choosing PNG24? If you are using something that has hill shading, then you probably won't be needing to have transparancy. Aerials and other raster based layers are best to do in JPG with a compression of 60 and a tile size of 256. Your goal here is to minimize the size of your downloads to the user, and PNG24 produces large file size, and takes longer to cache. If you are doing something where you need transparency, then I would try to do it in PNG8 since that will also keep file size down. The only time you would want to use PNG24 is when you are using anti-aliasing, which causes the number of colors to exceed what PNG8 can handle. Personally, I just use PNG8 for vector services, and JPG for rasters. Next, I'm curious why you have scales of 128,000 and 200,000, since normally you want to double the scale each time, though if 200,000 is good enough to cover your area, then I guess that could make sense...
So, to your question of caching only a portion. You can do this in a couple of ways. In ArcMap, the easiest way is to first set the visible extent to what you want it to be, and then right-click on Layers in the TOC and select properties. In the Data Frame tab click Other and then Specify Extent. Once there, click on Current Visible Extent. You could also explicitly set your extent if you know it. Also, while you're here, make sure the Coordinate System is correct. The other way to do only cache a portion is to do it when you are actually doing the caching. At the bottom of the Manage Map Server Cache Tiles dialog, you have the ability to "Update specific areas using a feature class" where you can supply a rectangular feature class that covers your area. I would choose the first way myself...
