If helmets and horns together are an issue, a possible very simple solution:
Horns ARE the helmet, meaning if the character is wearing a visible hat/helm/etc. then the horns are not visible.
Or a less quick-and-dirty idea: add a flag to each hat/helm graphic for whether or not they would have a problem being shown at the same time as head horns. Most head item graphics probably would, but some would not make interference.
Also some ideas about what Tiefling race might be like, without knowing anything about any PnP precedents etc:
Can't be Lawful Good (no Tiefling paladins)
Same build points as Drow
-2 STR, -2 CON, +2 DEX, +2 INT, +2 CHA starting stats
Racial Enhancement Tree: Can raise DEX or CHA each tier
Racial Enhancement Tree: Racial bonuses to these weapons: Hand Axe, Buckler, Light Mace, Light Hammer, Dagger, Throwing Hammer, Throwing Dagger, Throwing Axe
Racial Enhancement Tree: Can use DEX for damage modifier on weapons listed above (still need Finesse for melee attack mod though)
Other racial enhancements might include things spell saves, fire resist/absorb, chance to negate energy drain or stat damage, deathblock as aug, fortification or dodge+dodge cap, reflex saves