There is a specific number of Overseers for a reason though it would be spoilery to discuss why. Elidibus is a special case because he is an Emissary. Again, from later MSQ I get the feeling this role has a specific purpose. Both Zodiark and Hydaelyn have 'rules' they have to work by. They have to generally work through their respective agents.

We do learn from the SMN quest that lesser Ascians are individuals who have been 'raised up' by Ascian Overlords. We also learn that lesser Ascians don't have the complete set of abilities their masters have.

I feel this will again come back to the Echo. We honestly have no idea what the world was like before the first Era. We barely know anything about that period of time. Even where the MSQ is there is a lot not known and many things that have been revealed raise more questions. For the Ascians though, regardless of their age, I think originally they were individuals like the WoL with the Echo who for some reason or another chose to follow Zodiark. Likewise I suspect that realistically many of their abilities come as much from having a much greater mastery over than any on team Hydaelyn seem to have.