xml files can't program anything. they store data (and are there a from of database).
it's not clear what adding more buttons has to do with your xml file. is there data in the xml file that relates to buttons?
and it's not clear what images and a slide show have to do with anything.
but commonly, an xml file would store data about what images to display in a slideshow. and the xml file might even contain text to display for each image and possibly an external link associated with each image and maybe some specifics about how to display the image.
So...would you continue to add more data to that same file as it is telling Flash what images to load? Or would you duplicate that XML file and add new image names.
What's the working method with this - is it typically 1 file or multiple XML files that people work with (in general)?
that depends upon what you're trying to do and you're current xml file is doing. i just suggested what your xml file might contain but beyond being certain it is not programming anything and being certain it contains data, i really don't know what data it contains and i don't know what you want to do.
but if you have a slide show and you want to add more images to the slide show, add more tags to the xml and see if it works. but you better know something about what you're doing or you're going to screw-up the xml file.