After I discovered that the only device that uses the 1x Universal image asset was the iPad 2, I went back and did a little bit more experimentation. If the 1x image is missing, then the iPad 2 will use the 2x image, scaled down properly for the device. Since it is a easy 50% reduction in the image size, there should be no degradation of the image quality.
The iPhone 6 also adjusted correctly. If the 3x image was missing, it upscaled the 2x image to fit in our screen size properly. Although functional, upsizing the image will cause a bit of blur to occur, so I’ll continue to support adding the 3x images.
So the only assets that need to be used in a Swift SpriteKit application are Universal images 2x & 3x. This let me reduce the overall file size significantly and made my job a whole lot easier.