I think it would be better worded:
N/N
One Soul: N = number of living creatures in play divided by 2.
Anyways I like the idea, and I don't think its necessary to exclude skeletons. If I understand the mechanic correctly, the highest attack this creature could have is 23. And that would mean a 'living' creature in every slot on both sides of the field.
Now I do think that no Shadow Beast/Spirit should count in creature count. If one Shadow Spirit doesn't count for itself, I don't see a thematic reason as to why another Shadow Spirit would. Then on the calculations end I think it would be simpler to just say Shadow Spirits don't count in the total instead of explaining that whole thing about how one doesn't count for itself but another will etc.