What is your specific gravity?
I think the best star would be a Linckia multiflora, if you want a true "seastar." I think an orange linckia will probably be ok, but I would STRONGLY discourage a blue linckia or Fromia of any size. Also only keep one as they compete for food. These seastars can not be spot fed, but graze on surface microfauna on LR, and most will die in 12-18 months of starvation. People often think they are easy to keep but are unaware of their needs and have not kept them long enough.
However Linckia mutliflora has done quite well long term in smaller tanks, even reproducing via arm drop, so your tank is likely to be ok.
But finding the right specimen, and acclimation, are critical. If you have issues with your alkalinity, you also want to get that straightened out too.
As for the seahorse, I really, truly, do not recommend them in larger community reef tanks. I believe several species are highly suitable for captivity in appropriate tanks.