I’m not sure I understood your follow-up question, but I can add something regarding the initial question, I think:
If you use the slider, then under the slider settings in Graphene options, you can indicate what posts should be included or not. If you want to show only some posts, then the easiest way is to create a new post category “FeaturedInSlider”, for instance. Then instruct the slider to only show posts in this category, and next, make sure that the posts you want to show in the slider are included in this category.