I don't know about reapers in low-pressure-environments... then again, sectoids are pretty silly in that setting, too. If we want it to make sense, I'd go for cyberdiscs, sectopods, floaters and silacoids in the mars-surface-battle. those are the aliens that I'd believe to survive that environment. ...also, we should disallow anything below power-armor for the xcom soldiers
Although I think Mutons also have potential to survive, perhaps even greater than Floaters.
Mutons still have to breathe. Floaters have large parts of their body replaced by a life support system ("The device which seems to form the core of the body is a life support system, taking over the function of heart, lungs and digestive system. This would enable the creature to survive in extremely hostile environments."). I'd go for Floaters
what about sectopods? Too powerful? I thought that was the whole point of this thread, anyway