The elemental summons kinda suck (especially water), the only one that's somewhat okay is fire. However, almost all of them except earth has no changes between summons except for a small boost in stats. They all function the same, with little point in the low level summons since they just die.
Either buff the flat numbers some, like adding a bit more ac/hp/ab so they do something in a fight, and/or have there be different summons. For instance fire could have a hell hound summon that spams the flamecone and somethind that casts scorching ray, air could have a summon that uses balargn's and gust of wind and electric spells for ranged, etc.
I think each one should have an elemental mephit with +1 regen and 5-10/+1 DR, (or basic d&d +2 regen mephits get) somewhere within the first three summon spell levels.
Air dust ice and fire.
Having played a positivist to 9 and even 10 con wizard to 9 (3x over) with the Celestial theme I know you can formulate a LOT of varying strategies & combinations to deal with a variety of grouped up mobs, because of the variety of sniping, tank and damage reflection, various different creatures you can summon instead of a blanket-swarm of the same basic type.
A lot of the other summoning themes have creatures with concealment or DR that they can use, with undead types even getting a good advantage with negative healing support (at the cost of summoning replacement undead with the spells.)
Hellhound would do well too for fire, especially with burning hands / breath and alongside other summons adequately immune to fire, sounds very efficient and smart.
I disagree with the addition of a Hellhound. Hellhounds are Devils, not elementals. Their fire is literally from being from the hells not any elemental connection on their part. Mephits however are definitely a cool idea, especially since Earth already gets a mephit.