Have been doing some research & have some limited information on the temp/depth question. Most Synchiropus species are tropical & are caught in shallow water with temperatures in the high 70 to 80F range.
Then I looked at species with the most visual similarity to the Ruby Red. There are perhaps a dozen I saw and most had shallow habits & tropical temps compatible for our aquariums. They were commonly found in shallow rubble zones, lagoons etc.
The species mentioned earlier in this thread, Synchiropus Moyeri, looks closest to the Ruby Red to me. It's found in the reefs around the volcanic islands in the area south of Okinawa Japan. It's not a deep water fish, but the ocean temps there are about about 71F year round. Collected (S. Moyeri) at latitude 39 longitude 139, it is on the tropical/subtropical line, roughly. Looking at the map it seems entirely plausible that it is also found further south along other coral atolls, which might have even warmer water.
I would bet the Ruby Red & S. Moyeri are closely related and who knows, maybe overlapping habitats. So it's possible they might have somewhat similar requirements. And there are a few fish in the hobby that come from the cooler Okinawa sub tropical area that do well in our tropical reef tanks. So IF the Ruby Red were indeed a kissing cousin of the S. Moyeri, I would bet that a tank with temps in the middle 70F would be fine. I used to keep my tank at 75F but it's now about 79F so maybe I will do my maintenance ion my chiller and get it back in service. My other fish & corals won't care, so I can't see a downside. If my round-about reasoning probed to be correct and this fish likes the lower end of the popularly accepted reef aquarium temperature range of 72F to 82F, it might be more vulnerable to overheating accidents, but that to is just speculation.
So if someone can figure out where this fish is actually collected it's not hard to get an idea of the ocean temperatures there. I was wondering if Divers Den could get that information from their supply chain? But I'm confident that it's not a deep water or cold water fish at least from my limited research. Not like the Catalina Goby for example even though I'm sure the two fish could cohabitate. But unless we could get more information, I can't draw any firm conclusions.