A few Aiptasia rock anemones won’t pose a serious threat to any of the larger seahorses such as Mustangs and Sunbursts (Hippocampus erectus). The greater seahorses are quite impervious to the stings of hydroids, nor are they normally troubled by Aiptasia, which they can easily avoid.
However, Aiptasia rock anemones can rapidly increase in number and become a threat to seahorses when they are so numerous it is difficult for the seahorses to avoid coming in contact with them. For this reason, I would suggest that you take measures to control the Aiptasia now while there are still only a few of the anemones.
Aiptasia rock anemones can easily be killed by injecting them with a number of solutions — Kalkwasser, boiling water, lemon juice, a number of commercial products — but I suggest using biological control to eradicate them.
Peppermint Shrimp (Lysmata wurdemanni) will do a fine job of controlling Aiptasia rock anemones and they do great with seahorses. They are popular additions to a seahorse tank because hobbyists like to use them to augment their cleanup crews and add a splash of color and activity to their tanks. Aside from their utility as attractive scavengers, they often perform a useful service by grooming the seahorses, which is fascinating to watch, and regularly reproduce, releasing swarms of nauplii many seahorses love to eat. Peppermint Shrimp are especially popular because they are natural predators of Aiptasia rock anemones and do a wonderful job of eradicating these pests from the aquarium.
One rule to keep in mind when buying your Peppermints is to select the largest possible cleaner shrimp for your seahorse tank(s). Seahorses will actively hunt small cleaner shrimp and they are quite capable of killing shrimp that are far too big to swallow whole, so the cleaners need to be large enough that they are not regarded as potential prey. Just add a few good-sized Peppermint Shrimp to your tank, Mike, and your worries about rock anemones will be a thing of the past.
Best of luck with your new seahorse setup, Mike! You’re 65-gallon aquarium sounds beautiful and it will make a terrific home for your seahorses with all that live rock.