Is it OK to use zazen for the purposes one would use metta meditation? Because I don't want to do any meditation except zazen.
Generally, zazen has no content - only emptiness. That said, I'm not sure why you'd limit yourself like that. If you feel you need to do metta meditation, do metta meditation. If you feel you need to say amitabhas, say amitabhas. Zen training has no boundaries. Adherence to a strict set of practice rules isn't the point.