I think you could move all your rock over and have no cycle, just leave the tank bare bottom for a week or so and then add new or thoroughly cleaned sand from the other tank.
Plus, those ORA horses, especially the more exotic seahorses, like the kuda are really expensive. Surely you have a little cash laying around to give them a proper home. Look for a tall tank, they make them just like that, and it will be perfect for them. Call your local lfs stores, im sure they know who is selling out and where you can get one cheap or check out craigslist. These guys are so amazing, and so much fun, i just love mine, you really should give them the proper home so you can enjoy them for a long time.
But if you cannot get a new tank, i would suck out the sand to give them that extra inch, and stuff it with some pretty macro and hope and pray they dont end up with bubble issues. I would bet you call around you could find some of the pretty macros for free as well. Cheato is not going to cut for hitching posts. One coral that the do really love to hitch is the gorgs, but they are hard to keep in our tanks so before you buy make sure your system is ready for one, and research the one you do buy. There are different levels of difficulty with them. The browns ones are easier, the more colorful the harder they are.
Other seahorse stuff you should know - no stinging corals, no inverts that are going to grab their tails, especially watch out for the pest nems, they can spell death for seahorses because of the stings. I would go with a snail only CUC. They also dont always like bright lights, so make sure they have a shady cave they can go to.