My little bit of seahorse keeping experience:
24g should be plenty big enough for quite a few species, and indeed too large for the dwarf species. Seahorse.org is a good resource for tank parameters too.
Temp--yes, some species (Cape seahorses for example) are temperate and need room temp water, nothing above 72 (though I had good luck keeping SH fry slightly higher). If you choose a temperate species, you will need a chiller if it is kept somewhere not climate controlled, but there are "microchilelrs" available that are small hang-on units. Other speices are decidedly tropical and will need typical reef temps.
Definitely be careful about which corals you choose. No anemones, no stinging ones, nothing that has long
feeder tentacles. Xenia is a good choice. I see many tanks with Gorgonians, though I have not used them. And make sure to have plenty of appropriate safe "hitches" for the horses to hang on to.
Feeding--the dwarfs will need live food (BBS). Other than that, do yourself a favor and wean your horses onto frozen or other. For the small guys, frozen cyclop-eeze or (better) grated mysids are good.

Good luck
Christine