Bandwagon, here. Elemental, Small is how the Small Elemental is listed in the MM; its a particular set of creatures. You should be able to show that any X Elemental, Small entry works, but that only includes Storm Elemental, unless you can convince folks that this also includes the MotP's Paraelementals (Ice, Magma, Ooze, and Smoke). However, Storm Elemental's (MMIII) entry shows that even that reading is incorrect; Small ones being available via SNAIII rather than II, and SMIV rather than III, where each spell series' entries show 'elemental, small (any)'.
Now, you might be able to argue that PHB's rules trump MMIII's, and if you can show that the Paraelementals are summonable, they have no text that would contradict the PHB.
Endarire's position seems to be that since the Summon Elemental feat lets you "summon a Small elemental (air, earth, fire, or water; your choice)," you can actually choose 'any Small creature with the elemental type, of the air, earth, fire, or water subtype.' The capitalization shows that that's a Size Word, but perhaps not a creature name, but a type. This same reasoning would permit a druid to have any Small elemental (type) in place of its animal companion.
For the druid ACF, the context does not support this reading, as the elemental advances to Medium, Large, then Huge, which seems to be RAI as the creature named 'elemental.' The reserve feat has similar wording, except that you can't choose to have smaller elementals increase in Str/Con/HD rather than increase their size.
It's slightly more nebulous than I'd first thought, but still seems to be the specific creature, as each ability should be assumed to be specific, but the designers aren't always as clear as they could be.
Good luck.