Alright, I checked and looked them up. Here's the top beings in the various elemental planes outside of the cardinal 4. I excluded any of them that weren't elemental in origin, like the giant brine dragon in quasi-elemental salt, or the Greater Doomlords of the Doomguard in the negative quasiplanes.
Paraelemental Planes
Magma - Chilimba: The Searing Emperor, First General of the Cauldron, Master of All Mephits. He's a fire mephit.
Ooze - Bwimb II : Archomental of Ooze / Paraelemental Princess of Ooze. She's listed as a minor archomental, or as close to an archomental of Ooze as you'll likely find. Possibly allied with Juiblex, maybe having risen up from the corpse of her father Bwimb after he was plastered by Orcus/Tenebrous.
Smoke - Ehkahk: The Smouldering Duke. A powerful smoke mephit.
Ice - Cryonax. He's a legit archomental it seems. Unique form, and fathered by the Elder Elemental God supposedly, which is a common trait of the other evil cardinal archomentals.
Quasielemental Planes
Mineral - Crystalle : claims to be an archomental, appears to be a mineral quasielemental. Seems to be good aligned.
Radiance - none really. The enigmatic, birdlike beings of godlike power known as King Black and Queen White of the Refuge of Color are the closest to rulers of the plane, and they're largely unconcerned with non-natives, or anything beyond their subjects the Varisoh on the one spot of 'natural' solid ground in the plane.
Lightning - none
Steam - none ('cept Xanxost
And Xanxost is a slaad and shouldn't be in the inner planes. Or so he says since he wrote that section of the book IC)
Ash - none
Dust - Alu Kahn Sang : High General of, or the Wind of Destruction. Powerful dust quasielemental.
Salt - none
Vacuum - maybe. Sun Sing is an unknown being of massive power living within a pocket of negative energy in the plane, somehow surviving. Rumored to be the last surviving Void Mephit, now of the power of an archomental. The other rumor is that he/she/it is a lich, demilich, or other massively powerful type of undead being. None know for certain.