I use a HOB filter with my seahorses stuffed with live rock rubble. You should be careful when adding live rock into the tank because you need to feed seahorses heavier than regular fish and they eat slower too. Bristle worms in your live rock begin to grow rapidly and will venture out during feeding times...which can cause problems with seahorses. If they get too close to a bristle worm while feeding, they can get some of the bristles in their snout which can cause snout rot. In a 10 gallon if she would like a skimmer...I would say a fission nano would work well, but a skimmer is not necessary. As far as corals go...anything that does not have stinging tentacles is fine (naming those to avoid is quicker...bubble coral, euphyllia, ANEMONES, sun coral). Macros are great for color and hitching posts, they also provide a home for copepods and other bugs to live in and the seahorses will snack on those. The brighter the decor is...the more color displays the seahorses will show (especially during mating).

NOTE: Watch your intakes on filters and pumps!!! If the seahorses are young their tails may go into the intakes and damage their tails!! I cover mine with fine mesh bags and secure them with rubber bands...better to be safe than sorry!