In newer SVG standard, there is an attribute which is called 'non-scaling-stroke' that prevents scaling of the lines, which can be used to model scalable graphics with outlines.
So far as I know, in current version of yEd, there is no way to scale imported external graphics with non-scaling outlines, since outlines become also thinner or thicker with the shape inside.
With this feature, the shape of the palettes would be greatly extendable with external SVGs! I wish it would be supported soon. Thanks for reading.