Pirkko, you are totally allowed to resize, recolor, or otherwise modify ready-made kits. Designers kind of expect that so it will fit perfectly with your photo and your vision. As for artwork, yes, it is a muddy area as so many are used illegally and then distributed on “free sites”. It is really hard to know what is legal and what is copyrighted.
As for the workspace, you can customize it as you wish. I offer a suggestion but if your workflow is better with other arrangements, that is fine. At least, you know that you can customize it for YOU.