Easily registering WordPress custom post types and taxonomies
Of course it's ok. You will have 1 object per post type. GetInstance is called factory method.
I have noticed two strange things in your code:

PostType.php Line 116

Why do you use random here? It will be ok if you leave default priority param.

PostType.php Line 123
You call toSlug function before slug usage but it's better to call it when name was setted up here PostType.php Line 380


This way you will never forgot to call toSlug before slug usage.