Hi
In future please create new threads each time you have a new question, you have asked several different questions on this one thread which are not all relative to the title. Making it hard for other people with the same issue to find the solution.
To only apply the custom menu to certain pages then all you need to do is assign the sidebar with the custom menu to the pages that you want it on, otherwise assign a different sidebar.
You can’t reduce the width of the sidebar because it is built on a 12 column grid, you would have to hack the files, which will mess up the layout of your pages.
– Kyle