If you only want specific threads to show up, then it is best to create Custom Content items for the threads you want to show up, and then do not include the whole forum in your forum selection, if that makes sense. There currently is not a way to select only specific threads. The slider pulls the most current (quantity defined by what you set in the settings) content types from the various areas: CMS, Blogs, Forums. If you have 2 forums selected, and 1 is more active than the other, then most of the content in the slider will be from the most active forum. And also, if I am reading your post correctly, you are wanting to turn off the content area in the slider (to make it more like a slideshow), then the only way to do that as well is via the Custom Content items. When you create a custom content item, put in a title and an image, but no message, and the slider will only display the image.
As far as the CMS goes, if you have the CMS disabled (via Product Manager), then the slider shouldn't be pulling any content from there.
Hopefully I understood and answered your questions.