If the calculated load of your 400 amp service is 380 amps or less then 500 MCM copper is fine but if your load is 381 amps then you would need to use 600 MCM. The same goes for the 600 amp and 800 amp services. As long as the calculated load does not exceed the conductors rating your ok. What do you mean by "I believe the neutral should be counted"? If you are saying that the neutral should be counted for derating purposes I am not sure why. If this is a delta service the neutral is only carrying the unbalanced load of a single phase winding on the transformer so harmonics should not be a problem.
