Really, I am not to sure about Nagini in the Horcruxes time line. I always thought she was before Harry, and thats why she was able to help find the "main soul" part.

I thought that the Horcruxes were parts of the soul that anchored the “main soul” to this plan. It was like their first job so to speak.

What I thought was odd was, other then the dairy soul, none of the others attached to living things, tried to take over. It might be because the dairy souls was the largest because it was the first. However you would think the Harry Soul would try to take over.