I am not sure if this is what you are looking for, but Mendix allows you to configure validation rules for your attributes.
Just right click on an Entity in the modeler and navigate to validation rules. (see screenshot)
Hope this helps,
I don't follow, you have a product entity that is associated to a category entity. Because of this fact any value within the category will be shared through any product associated to the same category.
If you want to make one piece unique you add an attribute to the product. You can copy an attribute from the category and stamp that value into an attribute of the product as a default, then allowing the user to change that value.