I'm think the seahose people use a taller tank for anything other than the dwarf kind. That's why they don't use a regular 10gal. Not sure how tall the cube is?
I've bred seahorses in the past. I really wouldn't tend to recommend them for smaller tanks based on the amount of food they require. They have an extremely short and inefficient digestive tract and thus require a fairly constant supply of food for long term success. Assuming you get them onto frozen mysiss and euphausids you should still feed them at least two times per day if you plan to have long term success with them. I used to feed my brood four times per day. If you plan to keep seahorses you should also keep in mind the commitment you are placing upon yourself. It can be difficult to find people willing ot make sure your ponies get fed while you're on vacation etc.
Also, because of the amount of food they consume I wouldn't really recommend them for a nano with corals. You'll likely run into nutrient problems.