Well, I think these dragon demons would have tried shifting to human forms, after all, it seems that humans are applenty. A human form would give them a lot of advantages over their dragon form or their original allien form when dealing with humans.
Also, I think, if you have the ability to shapeshift to many forms, you would not limit yourself to shapeshift to human. You would certainly try other forms like say a bear, a dragon, an eagle or some other cool creatures. I was actually wondering about it when reading the book. It just doesn’t occur to Tom to shapeshift to other form, he doesn’t even try.
As for cats, large cats like tigers would be a better choice than you house pet. They are bigger than humans.