Major problem that I have with fallen celestial templates is their tendency to increase the base creature's power. I don't think that a fallen solar necessarily should be any more powerful than one who sticks to the straight and narrow.
Also, a fiend is a perfectly reasonable class of fallen celestial. Judeo-Christian tradition regarding demons separates them into three major classes: Demiurgical creations (apocryphal, but there is the suggestion that there are "demons" running around even before the Word brought forth order from chaos); the Lilim; and the fallen angels of the Rebel Host. So, while all demons don't need to be fallen celestials, fallen celestials certainly make up a large portion of demonkind. It seems fine to me that a balor would be a celestial perverted to the cause of evil.