Dolphin chariot is good idea. Though it's possible that it won't add so much to Oceania specifically. According to MA idea of more advanced technology, they could gain some harpoon-throwing troops, though...
Considering Ammonite - it could be cool idea, but unfortunately, this would be trading much movement for some more protection. And fossils are out of question here, as such fossils would only contain shell - so nothing to reanimate...
In any case some type of Kraken mage summon would be cool for this purely aquatic race. Another possibility could be well-armored giant slug(s) with some special effects or crabs which could leave water...
Considering swamp survival: this is good idea in my opinion, but maybe not so appropriate for ichtycentaurs... Maybe some special unti/summon (like the above-mentioned slugs) could be better.
Another beast often forgotten is hyppopotamus - that's quite amphibious creature (though it breathes with lungs just as crocodiles do) with trampling and very healthy bite, though it's somewhat smaller than elephant (size 5?).