Apparently Spine2D can't separate a skin with more than 1 category which means I can't sort the boobs size separately from the skin color.

You may have to manually code the "boob_left" and "boob_right" slot to show the boob size with the name "boob_(left/right)_(skin name)" . Luckily Spine 2d only shows 1 layer per slot so only one boob size can be shown.