Those tankmates would potentially be too aggressive and outcompete for food with a large seahorse, but they are definately life threatening for a seahorse as small as the one you are describing. They will eat it. The color of the seahorse does nothing to help identify the species of seahorse since seahorses can change color. Look around the gallery at seahorse.org
http://gallery.seahorse.org/main.php and see if you can identify your seahorse. If it was wild caught in florida, it is most likely either a dwarf seahorse (hippocampus zostrae) or a southern erectus seahorse (hippocampus erectus). A seahorse that small doesn't just need brine shrimp, it needs newly hatched baby brine shrimp, so be aware that you will be constantly hatching baby brine shrimp. Wild caught seahorses also need to be treated on purchase, as they come in with more bacteria and parasites than captive bred, so they are more fragile. Here is an article on caring for dwarf seahorses
http://www.seahorse.org/library/articles/dwarfKeeping.shtml, and here is discussion thread on the special needs of wild caught seahorses
http://forum.seahorse.org/index.php?showtopic=5946. Also, here is an article on appropriate tankmates for large species of seahorses, however be aware that anything ranked higher than zero poses a danger to baby seahorses and no fish are appropriate tankmates for dwarf seahorses.
http://www.seahorse.org/library/articles/tankmates.shtml
Lastly, seahorses drag their tails along the sand, rock, and anything else in the tank, and will also try to hitch to pretty much anything in the tank, making your aquarium very dangerous for them as you have stinging corals, especially the plate coral, that could easily kill this seahorse.
If the seahorse in question is hippocampus zosterae, it could be kept in a very small aquarium (2 to 5 gallons), provided it had no other tankmates and received the proper care.
When people talk about seahorses, they are talking about adults of the larger species of seahorses (4"- 8" or more), and a seahorse that is only 1/2" in length would require special care as either a dwarf seahorse or a baby seahorse in a dedicated rearing system.