Okay so an anonymous commenter remarked that the tiefling seemed a little powerful. Accordingly I've decided to run it through the ACKS Player's Companion class construction system, which I think is probably the best resource for that kind of thing. (Those of you who aren't familiar with it, just nod and smile). This is what I uncovered:
First of all let's say that the Tiefling racial abilities (immunity to fear, three thief abilities, and magical ability) are roughly equivalent to that of an Elf. My rationale for that: a hypothetical value of Thievery Value 1.5 (4 skills, assuming we trade off one for the fear immunity) would be 300, since Thievery Value 1 is 200 with three skills and Thievery Value 2 is 400 with five skills. Plus Arcane Value 4 (2500 and full caster progression) brings Tiefling value 4 up to 2800.
It has a d6 HD value, so that's another 500 for an XP base of 3200. Add a fighting value 2 (equivalent to the fighter class, 1000 XP base) and you walk away with an XP value of 4200 from first to second level.
Okay, so it's a little more powerful than an Elf at this point. To that end, let's tweak it a little bit and bring it down to a hypothetical Tiefling 3 (the equivalent of an Arcane value of 3), giving it a spellcasting ability of a mage equal to 2/3rds its level. So its arcane value adds 1875 to the XP base instead of 2500. 1875+300=2175, 2175+500=2675, 2675+1000=3675, in other words very slightly less than that of an Elf. For convenience's sake let's suppose that I want to try to get a little closer to an elf's progression, so let's bump up the Thievery Value all the way to 2 and give them another custom power, something like the Zaharan's ancient pacts. 1875+400=2275, 2275+500=2775, 2775+1000=3775... and honestly I think that's close enough for me. So the Tiefling will be revised tonight.