most fish are competitive for food, but it is weird that he doesn't hide. they're named dartfish because they dart back into their hiding place when they feel threatened. Maybe he's just really comfortable with you.
As for the non-aggressive eater thing, here's my opinion... I think that only applies when you feed the fish by just dumping the food in... However, if you have a feeding station for the seahorses, you can probably have any other tankmate (as long as they're not aggressive at all). I read about a great idea for a feeding station on another website. here it is: you take a clear petri dish, and put it on the bottom of the tank. You put a little sand in it, so the sides of the dish still stick up, but the sand will weigh it down. This becomes your feeding station. That way, you just put the mysis inside of the dish, and the food will stay in place while the ponies eat. When the ponies are all done, you can just suck up the uneaten food with a turkey baster. That way, you're not overfeeding, and the food isn't getting blown away before the ponies get it. It's also clear, so it's barely visible. I'm actually thinking about doing this, because my feeding station doens't work that well, food still goes all around the tank.
as for a cleanup crew, you can add just about anything. I wouldn't add any kinds of star... brittles have eaten slow swimming fish, cc stars will eat anything it can get ahold of (so i've heard), and other stars don't do well in that size tank.
i wouldn't get turbo or astrea snails, unless you can easily access everywhere in your tank. the reason i say this is because these types of snail can't un-flip themselves if they land upside-down. This can cause water quality problems. You can add hermits, nassarius snails, cleaner shrimp, fire shrimp, and peppermint shrimp. I think those are your best bet. (don't add margarita snails, because they require colder water). you can also add some sort of crab that has rounded/blunt claws. The best is a porcelain crab. They are really kinda filter feaders that sift through the sand for little pieces of stuff to eat. They have rounded claws and are incredibly peaceful... Make sure you don't get any kind of crab that is territorial (such as emerald or sally lightfoots), or any crab that has sharp pointed claws.
Also, make sure you research the kind of seahorse you want. Some require cold water. The best beginner ponies are h. erectus and h. reidi.