Hi,
Imsanity plugin optimizes the actual image files. For example, if you upload a 5mb image that you just took with your camera, Imsanity will reduce the size to something sane that can actually be used on the web.
My plugin is not concerned with the actual file size. What my plugin does is reduce the number of files each image upload generates in your uploads folder (unless your theme/plugins don’t define custom image sizes, which is highly unlikely). So by default, when you upload the image to WordPress, it will generate at least 4 files from that (thumbnail, medium, large and original). On top of that, your theme probably defines custom image sizes it needs, so those are generated as well. Same for the plugins that do some kind of output on the frontend and use images. My plugin stops that from happening by default, and instead only generates those sizes if (and when) they are actually needed.
Long story short, you can use both plugins simultaneously.