Zen sub-themes, sass vs inheritance
see this issue queue How to Approach Sub-Sub themes with Zen?

You dont copy zen starter kit for every sub-theme. You basically copy the .info file and template.php and create your theme folder. If you copy zen starter kit for every theme you'll run into the problem your seeing you end up "spamming" css files everywhere.

Sub-themes are best built smallest to biggest. By which I mean start off with the smallest number of .info file attributes and build up to the subtheme you want.

With so many subthemes you should be able to come up with 1 generic "base sub-theme" you then base all your subthemes off of in code.

Every sub-theme needs 1 uniquely named CSS file to properly fire off inheritance.