Pretty certain more powerful dragons cast spells, a bit iffy on the weaker ones. But for shape shifting familiar shapes are easier. As former humans the best shape would be themselves or somebody they new well, pretty certain Tom was imitating somebody he used to know as opposed to an imagined human. Imitation is easier than creation, so ideally you would imitate a cat you knew well. Also forms closer to the demon are easiest. A large demon does not do as well imitating things that are to small. A cat is more compressed than a human and more foreign to the demon. So demon cat form will hurt a lot, but Bess could likely do it if she was inclined to try it. It would be easier for her than other demons with her cat motif and everything.