This sounds like something that is interesting for me too… I am struggling with the same problem in flashlight theme. I don’t want to show the gallery image (used as a sorth of featured image.) in the post itself..
You suggest to add a category No Slider, but I use the portfolio categories.. do I add the php code in loops-index.php in the same way or are the portflolio categories different from the normal post categories? regards, Marja