Sure Pete, feel free to use any info you’d like!
I’ve found a bit more info about potential tankmates, too. Bob Fenner at wetwebmedia notes that the Blue Spot Jawfish is typically found at temperatures in the 50s and 60s in the wild. So, I’d estimate that this jawfish would make a good candidate as a potbelly tankmate. It is another fish like the catalina goby that is typically sold by suppliers as a tropical fish. If adding this fish to your setup, be sure to add a few inches to the height of your aquarium. Blue Spots require a deep sand bed for burrowing. A deep sand bed added to the needed height for h. abdominalis would require a very tall aquarium.
ORA captive breeds three temperate fishes. The Hulafish that you mentioned as difficult to obtain in the states can now be obtained pretty easily, as ORA captive breeds them: http://www.orafarm.com/product/eastern-hulafish/
The other two coldwater fishes that ORA breeds, I don’t think I’d personally feel comfortable keeping as seahorse tankmates, but this is just my guess.
There is the Kamohara Fang Blenny. The bite of Fang Blennys is apparently venomous. While the bite isn’t especially dangerous and they’re not very likely to bite, I’d rather not risk it: http://www.orafarm.com/product/kamohara-blenny/
There is also a Pygmy Filefish that they offer that comes from colder waters in Southern Japan. Apparently, most filefish tend to bite onto things including seahorses not out of aggression, but out of curiosity, etc: http://www.orafarm.com/product/pygmy-filefish/
A few other fish that have come through Coldwater Marine Aquatics that I’d guess would do well with seahorses are Rhinogobiops nicholsii and perhaps the tiny Lethotremus awae as they appear to be weak swimmers. I’ve also seen a few people with Rhamphocottus richardsonii in their coldwater tanks. If they can be obtained, I would think they would do well, too, as these cute little fish appear to just slowly scoot around the bottom of the tank. Let me know if you think any of these fish would not make good tankmates if you have any experience with them.
For people who are experimental and willing to try to keep their seahorses with more aggressive feeders while committing to target feeding their seahorses (and to move the aggressive feeders to another system should problems arise), there are some other lovely fishes from a genus or two that we typically associate with more tropical waters, but that are species that they themselves occur in colder climates (down to at least the upper 50s) such as anthias like Sacura margaritacea and Anthias anthias, Callanthias japonicus, and Caprodon schlegelii, and butterflyfishes like Chaetodon auripes.