Catalogs contained completely separate references to images. If you decide to create a second catalog for your stock photos and clipart, and don't have any of that data in your original catalog, then everything will be completely separate. If some of your images are in both catalogs you could end up with different versions of the same image because one catalog would be aware of the adjustments made in the other.
Generally speaking, it's usually best to work with a single catalog. However, in this instance, it might be justified if you are trying to separate stock photos and clipart from your personal photos. There really isn't any right answer. The decision is yours. And either way will work. For instance, if you decide to keep everything in one catalog you could put all of your clipart and stock photos in a separate master folder and still keep them somewhat separated.