Regarding the powerheads, the Hydor Koralia units are great, but in my experience, even a Koralia-1 would most probably be too much for a 30-gallon aquarium. I very much like the Hydor Koralia powerheads for large seahorse tanks because of their unique "egg" design that surrounds the mechanism with tiny slits that are too small to allow the tail or snout of a curious seahorse to enter. (With other powerheads, it’s very important to screen or shield off the intake for the powerhead so that a curious seahorse cannot injure its tail or snout on the impeller.)
Your proposed arrangement for the powerheads sound sensible, Dawn, but I think I would only use the nano powerheads. Placing a small powerhead so that it roils the surface of the water to provide good surface agitation and better gas exchange at the air/water interface is a good idea, and I think a nano should be sufficient to assure good water circulation along the bottom. But you’ll have to wait until your live rock and decorations are in place to be sure so that you can determine whether the rockwork creates any "dead spots" that should be addressed when positioning the powerheads.
Best of luck cycling your new seahorse tank and preparing it for the ponies, Dawn!