Myriad : don't know, sound like it belings to an application. I bet that you can remove it.
Multiprocessing API Library: if you don't have a multi-processor Mac, trash it without worry.
If you are running OS 9, many descriptions are available to you from extentions manager.
As for moving things into their own folder, I think that is ok. Try it an if things go awry, put 'em back.