What app do you plan on using to create a slideshow? If it's Photos then the image files should be imported into the Photos library before they can be used in a slideshow.
Once imported into a "managed" library any time you need an image file for use outside of the library you can select it and export either the original file or edited version to the Desktop. This allows Photos to manage the files via a database, maintain nondestructive editing, display the photo in multiple views all with only one original file on the hard drive.
One can keep the image files outside of the Photos library and still use them by running a "referenced" library.
However Photos is not good at running a "referenced" library. If an image file or folder of files is moved it will break the link to the thumbnail in the library and corrupt the library. This user tip described other pitfalls of a reference library: Disadvantages of a Referenced Library in Photos for Mac
If you keep your photos in folders and want to create a slideshow in a 3rd party app you first need to search all of the folders for the images you want to use, copy them to a folder for that slideshow and then create the slideshow. This adds multiple files to the hard drive for each image. Somewhat inefficient.
To help you search your folders for images there's a file browser that may help: Lyn. With it you can view all of your photos on your Mac in a browser and it will let you rename the image file and add EXIF and IIPTC metadata from within Lyn. However, to group selected images in another location for use by another 3rd party app they will have to be copied and moved to that location.
Here are some user tips from Photos for Mac User Tips that will help you in your transition from iPhoto to Photos:
Moments in Photos are the New Events.
External Editors in Photos Are Here
Notes on Migrating an iPhoto Library from iPhoto to Photos for Mac
Photo Editing Extensions for Photos for Mac
