Is it permissible for a non-Muslim to testify against a Muslim in personal status matters, and is it permissible for him to be appointed as a legal guardian over a Muslim child, despite the presence of righteous relatives of the child?

The fundamental principle is that a witness must be a just Muslim, based on Allah's saying: "And bring to witness two just persons from among you," and His saying: "O you who have believed, if there comes to you a disobedient one with information, investigate." The testimony of a disbeliever is not accepted except in cases of necessity. As for appointing a non-Muslim guardian over a Muslim child, it is not permissible, because a disbeliever has no guardianship over a Muslim. Guardianship is valid for a free, sane, just Muslim, but it is not valid for a disbeliever because he is not among those qualified for guardianship over a Muslim.