Storme said:
No one can count on my feeble memory, but...
Isn't small elemental ON the list? Along with Mephit and others?
Yes.
There's a list of creatures you can choose, restricted by your alignment. In Core, it includes Shocker lizard, Stirge, Formian Worker, Imp, Pseudodragon, and Quasit. You can't take the Imp, say, unless you're LE, NE, LN, or TN. You can't take the Pseudodragon unless you're LG, NG, CG, LN, TN, or CN. You can take a Shocker Lizard or Stirge regardless of your alignment.
It goes on to say that this list is not exhaustive, and: "Nor is the master’s alignment the only possible categorization. For instance, improved familiars could be assigned by the master’s creature type or subtype, as shown below."
Fire Elemental is shown as an option for a master with a subtype of 'Fire'. Humans don't have the Fire subtype, so based purely on the text in the feat description, a human with Improved Familiar doesn't qualify for a Fire Elemental familiar.
However, since it also says "The list in the table above presents only a few possible improved familiars. Almost any creature of the same general size and power as those on the list makes a suitable familiar.", there's certainly room for the DM to add creatures to the list and provide categorization criteria that would permit certain humans to qualify.
-Hyp.