I feel bad about this but I do have one slight clarification to make, I am always hesitant about forums like these and the types of responses that come up. Sometimes people are helpful, and sometimes they are just judgey which doesn't help anyone. I appreciate you expressing your concerns while also providing helpful advice. Currently I am running a 20XH and starting a 36G bowfront, the 13G widescreen is a hospital tank. Agreed that one could live in a single room, but that doesn't mean you would want to, but when you are sick a smaller room is fine.
My question about the damsel was legitimate, referencing the article on this website about seahorse-compatible fish, the green chromis is listed as least concern, and yellow tailed damsels are listed as "with-caution" so to speak. Given that both are types of damselfish and their behavior is so different, it led me to think (in my opinion legitimately) that the sapphire one may be OK at least with caution. I certainly would not add it without hearing of someone else's experiences with the fish in any tank environment. I am hesitant about the flatworm exit as I am with most chemical treatments, especially not knowing the active ingredients. At least when I was using Joe's Juice on the aptasia I knew that it was a suspension of calcium hydroxide, which is sparingly soluble in salt water (due especially due to the pre-existing [Ca]) and it is equilibrium controlled in its dissolution so it has no significant impact on pH and only posed a hazard to snails that wandered into it.
I did have the series of misfortunes with the seahorses, however, and I am not sure if it is best to add a new one with my current one when transferring to the new tank. The new tank is still cycling, and will be for another month or so, therefore this question is not too pressing, but there were aggressions between the last male and the new female, and that was after replacing very quickly the first female. It will be seven to eight months that this female has been without a partner when I am ready to get a new one, and I am not sure if it would be better to continue to keep her alone or bring in a new male.
If you have any suggestions and time enough to send them I would greatly appreciate it, and I will continue to research the flatworm exit before using it. Also, side note, I have tried vinegar, freshwater and boiling water in addition to Joe's Juice on the aptasia, no success, I am quite envious of your lack of these pests, but as I have also mentioned, they are not causing harm at the moment, so they are of concern, but not nearly a threat. Another side note, aptasia and nassarius snails will fight over freshwater mysis shrimp, so at least they provide some entertainment in that respect.
I feel a couple peppermint shrimp will not be too harmful to include in the new tank as an insurance policy.