I use method mobile first or a reponive webite and i use susy. How organize files _forms _typo _layout,_mixins etc.. for mobile (default) and breakpoints (tablet, desktop) and to have output files :
mobile.css
tablet.css
desktop.css
Thanks
Sass/Compass+Susy will create a file for every file in your source directory that is not preceded by an underscore.
So, simply place the code you want generated for those style sheets in files located in the source directory using any mixins from your own partials (files preceded by an underscore) or any of the extensions you're including on your project (such as susy)
A great document that talks about structuring your project is here: http://compass-style.org/help/tutorials/extensions/
Something else to look at: http://compass-style.org/help/tutorials/best_practices/