The Viking faction grew up for decades in the extreme cold of scandinavian nuclear winter. I think they should have a lower minimum temperature, or maybe in the long run they should prefer to colonize colder planets/moons rather than hot ones, or maybe create new species which can live in colder climates. Or all three of this, just for the sake of differentiating the factions a little more. Furthermore the vikings were historically great warriors but also great colonizers and explorers