Hi
First of all, I'm not Kenney. I'm just using his assets ;)
Now to your questions. The hat topic could be solved in different ways, depending on how much control you want to have. Do you want to have different hair styles below the hat?
If not, than I would maybe use the hats as a hair style version. The hair below the hat can then either be drawn together with the hat, or can be made a separate sprite as child of the hat with a lower sorting order than the hat.
Can you explain your question for the shirt a little more. I don't understand it fully.