1- Isn't this contradicting? Or are Google's suggestions in the first
guide are only meant for the creation phase of the code, rather than
the phase when it's already online?
No, is not contradicting. Those suggestions are only meant for the development files. The production ready files usually are minified since that operation can save a lot of bandwidth across all files you may have, let say .css
and .js
but then readability will be lost (which is not needed in production anyways).
2- Also, If I minify a file, do I have to change its ending from
style.css to style.min.css? I see many minified .js and .css files
that come like this. Is it vital, or is that just to indicate to
developers that if they want to modify the file, they better work on
the non minified version first, and later minify it on their own?
No, you don't have to, for instance, you can have only one set of names for your development and production files, but it is a common convention (a good one) to have .min.
files to easen the building and distributing of the files.