Here is my input:
A 26 gallon is only going to be large enough for one pair of seahorses.
Metal hallide lighting usually makes the aquarium too warm unless you have a chiller. Tropical seahorses like the species you listed need to be kept under 74 degrees. There aren't any species that can be kept over 74 without risking disease outbreaks, and some species need much lower temperatures.
Seahorses change colors so you shouldn't buy a seahorse based on color, as they will likely change to a different color once you add them to your aquarium.
Mixing seahorse species is not something to be entered into lightly. Different species of seahorses carry different strains of bacteria that they don't have symptoms to, but when mixed with other species, the other species catch these different strains and often get sick and die. With your setup, you only have room for one pair anyway, so I would just pick one species.
You'll be pushing it to keep even one mandarin in the 26 gallon unless you find a mandarin that is eating frozen. Most mandarins need a very large population of copepods to keep them from starving to death. This usually requires a very large aquarium with lots of live rock, or a very large refugium, neither of which you have. The other option is to culture copepods. The mandarin has even less of a chance when you put seahorses in, since seahorses will spend all day hunting your live rock for copepods, regardless of how much you feed them.
Peppermint shrimp are okay with seahorses. Smaller shrimp will be eaten by the seahorses, other species of larger shrimp can cause harm to the seahorses, and cleaner shrimp can irritate the seahorses into disease.
Remember, seahorses need lower flow than most corals. High flow will blow them around the aquarium. 5x turnover is usually recommended.
You will want to make sure you get captive bred seahorses from a reputable dealer like
www.seahorsesource.com or
www.dracomarine.org, because wild caught and tank raised seahorses bring in more issues to deal with in the form of parasites and disease, and a possibility of always needing live foods.
Oh, and you'll want the skimmer attached the the fuge instead of the main display to prevent microbubbles from possibly causing pouch problems with your seahorses, but having the skimmer on the fuge could interfere with the growth of your macro algae.