It's unusual to have that large a tank dedicated to seahorses, so your situation will not be typical. Nudis typically wander off into the filtration system if you WANT them to survive.
The problem with the tridacnid is that it will want a lot of light, and that it will close down hard on any creature that bumps its soft center. Horses are sometimes prone to wander into 'situations' and I would have a little concern for that, but you might be able to put some convenient 'grasping' branches just far enough away from the clam to attract them to that and not to the clam.
Sea apples and cucumbers, while fascinating creatures, are large and fleshy, and if they die, can emit a toxin [poison] that will do harm to the tank. I'd suggest more horses and pipefish, maybe a mandarin pair if you have pods enough: they're showy and will not harm your horses.