I'm pretty sure they aren't a species unto themsleves. Although the definition could be tricky; it's quite possible two tieflings would produce another tiefling (breed true, in other words), though it may have no resemblence to them (fiendish genetics being as they are...). They aren't a 'direct crossbreed', they're the offspring of a fiend-human pairing several generations ago.
Generally, tieflings have no place in normal socierty. They're too rare and unusual to truely fit anywhere. Treat them on a case-by-case basis. They make good thieve's guildmasters, cultists and conspirators, if it helps
On the other hand, a setting where they were more common could be intersting. A kingdom where fiends invaded millenia ago, for istance, may carry their taint in the bloodline, making tiefers a viable race; or the royal house may have married into a demonic family (!) for power, and the heirs to the throne are tieflings.
Looking at the stats, what can we say about them :
Fast, smart, antisocial/repellent, manipulative. Can see in the dark perfectly.
With cold, electric and fire resistance, they could live in the bitterest environments without complaint. In a steampunk city, f'rinstance they could live in huts on power lines or huge boilers without injury.
In a more mundane town, they could keep working through the heat of a desert midday while everyone else has a
siesta. They could go out in a blizzard to cut wood or hunt.
Interestingly, although they can see in any shadow or night or underground, they are just as vulnerable to each other's
darkness spells as anyone else is.
So :
* Could live in sewers or catacombs, benefiting from low rents
* With
darkness, could make good city guards, although they are genetically prone to corruption, and are more suited to the opposite side of the law.
* Excellent theives and conmen, of course.
* Probably live alone; shunned by normal society, they hate each other just as much a know
exactly how untrustworthy another tiefling is.
Well, there's some ideas. Hope it helps.
