Trek is entertaiment, so when it comes to costuming the real world need not apply. The concerns of a TV show or movie is different than that of a real military. TOS had four different uniform types (five if you count the pilots) The black collared shirts, jump suits, dress uniforms and the wrap arounds. It wasn't unusual to see two types together. I think the second TNG uniform popped up on DS9 from time to time. So I look upon the DS9 uniforms being in the same "group" as the TNG ones. Sort of like when my dad was in the Aiir Force. He usually wore his kahkis, but on occasion he would dress in his blues or fatigues.