CrPC mentions Officer in Charge and not a specified rank.General rule is "seniormost officer present". It is at least Head Constable generally. If in rare cases when no such person is present in PS then constables can register FIR and it will have the same effect. The details as to minimum rank authorized is actually as per relevant state police manuals.
Yes the registration is valid. The registration of an FIR is a part of an investigation and the law under section 156(2) says that investiogation cannot be challenged based on the ground of the power of the police officer to investigate.