If I want to serve these the same day, can I keep them warm in a slow cooker?
You absolutely can--up to two hours. Set your slow cooker to LOW and preheat it while boiling and assembling the potatoes. Melt a little extra butter and lightly brush the slow cooker insert. When you have the potatoes all stirred together, transfer them to your slow cooker and cover. Remove the cover occasionally to release any steam and condensation that builds up on the cover.