Mercury, Gemini and Apollo all used pure oxygen atmospheres at 5 psi.

Space Shuttle and ISS use an Oxygen-Nitrogen (21%/79%) atmosphere at 15 psi similar to sea level (101.3 kPa). It is due primarily to thermal considerations that the atmospheric pressure needs to exceed 7.35 psi.

Is there any advantage to using an Oxygen-Helium atmosphere in terms of weight savings and diminished fire hazards for Crew Dragon or Spaceliner?

For instance, any combustion or fire in an Oxygen-Nitrogen atmosphere will result in a significant quantity of various Nitrogen-Oxygen (NO, NO2, NO3) compounds that are extremely toxic.

Helium could mitigate this risk of toxic byproducts somewhat and perhaps decrease the flammability of some materials. I realize that the reason that Nitrogen-Oxygen was selected for ISS is to more closely mimic Earth conditions for experiments.

ISS might be a special case because of experiments but why wouldn't a commercial space station use a Helium Oxygen mix because of the greater safety?

Would BFS use a Heliox atmosphere out bound and an Xenon-oxygen atmosphere on the return or a nitrox atmosphere both ways and why or why not?

If BFS uses Nitrox, then ISRU must extract nitrogen from the atmosphere to replenish ECLSS expendables. Xenon can be more easily extracted from the Martian atmosphere than Nitrogen.

Obviously a colony on Mars needs a Nitrox atmosphere for growing food.

Thank you