I've got my package files all done, and everything inside them has a purpose. The accessory file is 871 KB and the clothing file is 1.81 MB. The 7z file that's holding them has compressed this down to a total of 1.42 MB. This sounds like a good compression to me, but I haven't squished a sims 3 file before. Is this the best compression I'm going to get, or is there some other compression routine I need to use? I've read about things like "Compressorizer" but don't know what it's for or where to look it up. I'm just trying to make this piece of CC as clean as I can before uploading.

That sounds fine IMO, the compressorizer is more for hairs and larger files, some hair files can get up to 10mb.

Open the package with S3PE and compress wats possible.

Also delete any duplicate textures (making sure that you know which ones are and aren't in use first!).