Answer:
Just to clarify. It is NOT yoorah Its Oorah. Second yes you can say Oorah after someone says Semper Fi. Oorah is used in many different contexts. Ex. Marine 1: Get some Marine! Marine 2 :Oorah Sir. Or You can say it in reverse like Marine 1 Oorah. Marine 2 Semper Fi.
Also Oorah comes after Marines are dismissed! When the Platoon Leader dismisses the Platoon the Marines yell OORAH!