Alright, here you go. My dream Zenki using the parts I listed earlier. See how the rounded body work all flows together? Disregard the wing, I have no idea what it is nor do I like it.
Attached is a Zenki with Kouki sides. They are rounded yes, but still don't match up with the car as a whole.
Second attached picture is an OLD rear shot (I have valences and exhaust now) of my friend and I. Note how the wing is contrasted by the bumper, and then imagine that wing on a Zenki bumper. No good. It would just be in the air for no reason.
Finally, my roommate's car. It is a Kouki with the stock Zenki wing but full JDM Kouki under that.
Hopefully that illustrates all the combos for you.