I'm cool with Tieflings...Evil is a means to an end really. As long as you don't disrupt the party, I'm not that strict on alignment.
Though that being said. Tieflings are evil by Nature, not evil by choice. Though many can become evil by choice; any Paladin i'd play would weigh the creatures deeds and concious actions before deciding if smite is really necessary.
As far as guns go, if you're a gunslinger, you'll have one, otherwise you need to be an arificer, alchemist, or 5 ranks in knowledge alchemy and enginering to conceptualize one. Of course if you take any of the gunslinger feats you will be able to as well. Firearms are nice, but really only benefit Gunslingers at the moment.
Callie, do you want me to tweak Tieflings? Though to be honest, I'm loving the idea of the Halfling with No Name or the Gnome Stranger.