It outright replaces the original CC content (requiring a savegame without its vanilla counterpart installed). Saints and Seducers is one small part of the ongoing Skyrim Extended Cut project.
Skyrim: Extended Cut is a nonprofit fan modification for Bethesda Game Studios' award-winning RPG title The Elder Scrolls V: Skyrim that overhauls the game's main questline, adding new choices, challenges and characters.
Would be great to see this Skyrim Extended Cut in it's entirety. But, as with 'Skywind' & 'Skyblivion' I will probably be pushing up daisies before the actual full release.