Hm. That's an interesting idea, but I wouldn't like it on a Federation starship. On B5 it made sense, because it was an interspecies diplomatic and trade hub. But Federation ships are those "action guys". Nobody should serve on a starship which atmosphere he can't breathe, simply because of all possible technical failures/invasions etc. It's like in the French foreign legion: everyone must be able to speak the same language to be allowed to serve together in a combat zone. I think: Every alien MUST be able to survive human conditions to serve on a human starship. But it would be an interesting concept for one-off episodes where they visit other starbases, or transport alien ambassadors on their ship.
That being said, whenever we will first see toilets in Star Trek (or even only toilet
doors), I want them to go completely bonkers with the bathroom signs!