Your suggestion present far more problems and questions than they solve... and those problems are numerous so I am not even going to make a list but instead I'll address your 'main reason':
my main reason for this is i hate having to balance a demon god to that of a normal girl JUST BECAUSE SHE HAS A RANDOM CHANCE OF BEING THE FIRST MONSTER CAPTURED... (over exaggerated!)
You see, this is the main problem when making girl packs for games like WM. In most Anime and Games out there girls are designed to be extraordinarily beautiful, strong, smart, scary, powerful, charismatic and so on. They also come from virtual 'universes' with their own rules and physics where those girls are strongest/fastest/prettiest etc. There is no denominator that you can use to compare all of the heroines and there will never be one so that means that your demon god power is very relative and by the same logic nearly all of girls in WM should have overwhelming starting 'Looks' stat and a large number of girls in WM should have insane battle stats from the start.
That as experience and reviews on forums has showed us is not always appreciated by community and doesn't make the game 'fun' for many players.
That can be solved in a number of ways as for example in Alkion we have decided not to allow player to set girl's stats in xml, instead starting stats will depend only on starting traits that you use to 'describe' the girl when you make a pack.
In the end, if you think for example that a demon god of yours should have high battle stats, another person will always come out and say that a different girl from some other game or anime will beat the living shit out of her and there is no way to come to a conclusive end in the discussion like that so we will get stuck with a bunch of overpowered characters like in WM unless that issue is solved on the design (code) level, but as you can prolly see, your suggestion doesn't really solve it, instead it encourages everyone to make overpowered girls as it introduces new ways to prevent player from acquiring them with ease.
PS: The 'main problem' really I think is that your idea is not actually all that bad BUT if introduced on it's own it will not make the game any better for obvious reasons and if you want to make a good game where girls have 'levels' and different rules for acquiring girls of different levels it will likely be a new game, not the original WM.