You cannot sort a slideshow easily, only be dragging individual photos around. You have to add the photos exactly in the order you want them to appear directly when you create the slideshow.
- Sort the smart album as you want it.
- Then select the smart album in the sidebar (select the album, not the photos in the album), then ctrl-click or right-click the album in the sidebar and use the command "Create > Slideshow > Photos".
This will usually preserve the sort album in the album for the slideshow. If you drag several photos at once to the slideshow, or if you use the "Create" command from the file menu, the photos may get rearranged. Then the sort order in the album will not be used for the slideshow.
One more thing. Photos will not completely respect the order you pick, when you use one of the fancy themes with several photos on a slide. The sequence of templates for the slides is fixed, and Photos will rearrange the photos to fill the placeholders with photos of the correct aspect ratio. If you want to control the sequence of the photos perfectly, use a theme with only one photo per slide, the simple themes "Classic" and Ken Burns".