Some things I notice in your composer.json
.
There is a version of that CSS minify available on Packagist which says it is just a copy of the original Goole-Code hosted files, but with Composer: natxet/cssmin. It is version 3.0.2, but I think that shouldn't make a difference.
mrclay/minify
is included twice in the packages with the same version. It also is available on Packagist. You will probably already use that (version 2.2.0 is registered, and because you didn't turn of Packagist access, it will be generally available for install unless a version requirement or conflict prevents it).You are trying to download a JAR file (which is a java executable without and PHP), but try to get PHP classmaps out of it. That will fail for sure.
You did miss the big note in the Composer documentation saying that Composer cannot resolve repositories mentioned in sub packages, only in the root package. That means that whatever you mention in your
alle/asset-merger
package will not be used if you use that package anywhere else. You'd have to duplicate these repositories in every package in addition to adding the package name itself as "required".
What this means is that you probably avoided missing mrclay/minify because it is available on Packagist, you might as well have added the cssmin by accident, but you definitly did not add YUICompressor.
But you shouldn't add this in the first place, because it is no PHP software. You can however add post-install commands to your projects. All your Composer integration does is download the JAR file. You can do that with a post-install or post-update command. See the documentation here.