You actually can make them slimmer since the bone structure doesn't rely wholly on the width of the waist. However, the hips and shoulders would need to be taken into account. I've made a lot of 'skinny' mods for skeletons like this anyway, and so long as you don't make the hips or shoulders significantly narrower, you can get away with it.
If DMC 4: SE is still moddable and these costumes don't appear, I'd like to make them. It's just a matter of finding time since I take the quality of my artwork fairly seriously (I wouldn't want to make anything less detailed than the existing models).
If DMC 4: SE is still moddable and these costumes don't appear, I'd like to make them. It's just a matter of finding time since I take the quality of my artwork fairly seriously (I wouldn't want to make anything less detailed than the existing models).