Luther , as long as the mapping coordinates on the hat stay the same, there is no problem with the texture being part of the coat texture.
Second question, yes please
. Most of the files I recieve don't use the same scale as my model, so having the body makes it easier for me to match the size, I just scale it up or down to the height of my character, and then delete one of them.
Last question: no, you can't do that
. The texture for the head extends to the last edge you see in the mapping coordinates. I made the division there because I wanted a round T-shirt collar, something standard, if you need anything else you should make it part of the model.
If you want to know where exactly these divisions are made, look for the material IDs in the model. When you are selecting faces, there is an option to select those Mat IDs, that will highlight the zone.