The question isn't really about whether navies (modern or futuristic) have alternative uniforms, though. It's about the [in]consistency of the presented universe. If the Enterprise crew wearing the DS9 uniforms had been phased in during the last couple seasons of the tv show, all these threads that question the mismatched look in "Generations" wouldn't keep coming up on here every few months. People would just accept it better. Instead, it does tend to raise questions, even though the answers are actually very straight-forward.