One very timid and peaceful fish is the best to keep with seahorses. I'm keeping a Yellowhead Jawfish with my seahorses. It is very peaceful, it doesn't bother the ponies, it isn't too aggressive of a feeder, and is easy to care for. That's a good option. Orange Firefish is very similar to the jawfish in terms of disposition so that's also a good idea. The Purple Firefish is slightly more aggressive than the Orange Firefish so I can't say how it would react with seahorses. I think the jawfish is my highest recommendation, but only because it's the only fish I've ever kept with the ponies. If you do that, just make sure that the tank finished its cycle before you put it in and that the jawfish has at least 3 inches of sand or crushed coral (sand's better but mine's doing very well with crushed coral alone) so that it can dig its tunnels. I have seen clowns being kept with ponies but I think it's a little risky. Clowns are pigs and so they'll probably eat up most of the food before the ponies can get to it.
Forget about clams with seahorses. Seahorses like dimly lit aquariums and clams need bright lighting. The only coral I would recommend with seahorses are zoanthids and some peaceful species of mushroom. Most coral have powerful stings, strong enough to kill seahorses, which is why you rarely see them in reefs. Nonetheless, you mentioned that you wanted to mimic the seahorse's natural environment and coral aren't really part of it. Their habitats are made up of lots of plants and very few pieces of live rock.
Since you're considering a 24 gallon, I would recommend only keeping a pair of seahorses. The more ponies you have in a tank that small, the dirtier it will get since they produce a lot of waste. The more waste they produce, the more ammonia problems you'll have. I only have a pair with the jawfish in my 30 gallon and the pair always turns a pinkish color in the morning and hold on to each other's tails. The male's never been pregnant, but it's nice to watch the seahorses show affection for each other. They are very fascinating to watch, and you don't need to have a lot of seahorses to enjoy the tank.