In a world like theirs', being a human (Even a smart human) becomes more and more of a weakness as the days go on.
He wasn't all that physically strong or coordinated (remember the dodgeball episode of the show).
Wasn't there an episode showing the opposite though? I think it was the episode where Dexter's dad was trying to get Dexter into sports and teach him. DeeDee was doing very well the entire episode and looked like what his dad wanted in Dexter. However, at the end of the episode, it showed him being very well coordinated and athletic with no enhancements of any sort (I specifically remember the handball). I could be remembering this wrong, but that episode always seemed to stick out to me, among others.
The PPG went through a tough childhood beating up monsters and I don't think they ever got well at all with people other than those with super powers. I don't think they would agree to put another young, powerful girl through a childhood even worse than theirs (trapped in a machine, forced to work for good at the expense of personal development and maturity).
They got well with the Pokey Oaks kids and that one girl in the neighborhood. Even the people of Townsville to an extent. I doubt their childhood was so terrible (excluding the close calls they've had). Though I agree that they would have reservations of putting anyone in this situation given the side-effects.
To add, in the movie it was shown that they weren't very well liked or trusted at first, but once they started realizing their responsibility and defeated Mojo Jojo and his Ape creations, they ended up loving the girls in the end, wanting them to stay and continue protecting Townsville. I don't think they would condone doing this to Mimi either, but who knows.
As for the cyborg part, is no-one considering the possibility that he just wanted to be one and there was no major ulterior motive behind becoming an automaton other than it being the natural next step in robotics, along with his extensive experience with cyborgs and fusing mechanical things with human flesh; such as his sister's android?
This I generally agree with. Along with wanting to catch up to those with superpowers, maybe he just wanted to?