What's your AC at would be the main question with Demons. More of them spawn at higher AC levels, and they get bonus starting XP based on the current AC as well.
Demons are meant to finally provide a good reason to want the AC maintained, at least until you can make peace with them for yourself.
The AC is in the mid-50s. I wouldn't call that a higher level.

I see what you are saying, but I still think Manticores with 3 promotions this early in the game (I just started getting Tier III troops) is a bit much.
Then, there is the matter of why the AI is pumping out barb Manticores, but I cannot get one despite researching the required tech?
I did notice that the creatures coming out of the Planar Gates for me (mostly Chao Marauders, Revelers, and Succubi) have 3 promotions too.