Looks good

@Background Pony #8902  
Angeal and Genesis would be individuals whom the changeling cells rejected, causing their bodies to go into degradation due to a lack of stability in their molecular structure.
 
Sephiroth was the only true successful result, being that he was given alicorn status since birth. But due to his lineage, he was unable to donate blood to others, even his friends.
 
Ancients (including Aerith/Aeris) were all members of the alicorn race, meaning that they were more understanding of the concept of magic itself.
 
But to further my explanation of Jenova=changeling:  
Jenova was considered an alien life form (capable of flight and magic) who took on the forms of lost family members and friends (shape-shifting). It took almost all of the ancients to take her down and to seal her away. As it was later discovered 2000 years later, they found her in a geological stratum.
 
TL;DR: I could write a fanfiction with this but I am too lazy to do so.

@anonymous-pony: What about Angeal, Genesis, and Sephiroth?

I hear that Jenova would make a good changeling.