Straight from the horse's mouth:
Creatures with zero Hit Dice... like tieflings... NEVER gain their racial skills or weapon proficiencies. Their class skills and weapon proficiencies are ALWAYS determined by their class unless it specifically says otherwise in their specific racial traits (such as how tengus gain access to all swords). The generic creature type class skills and proficiencies with weapon and armor never apply if the creature has no racial Hit Dice.
Hit Dice has EVERYTHING to do with class skills, in other words.
Humans have d8 Racial Hit Dice, but they don't gain the humanoid skills automatically as class skills. Tieflings thus don't gain the outsider class skills automatically either.
As soon as a monster gains a racial Hit Die... he gains his type's class skills and weapon/armor proficiencies and all that. Regardless of what class he ends up being. Which is why allowing PCs of monsters with racial Hit Dice (like gnolls or lizardfolk) is tricky, and recommended only for advanced GMs and players.
Oh, well, no scimitar for my aasimar...