Sounds like you are using "dwarf seahorses" as a generic term and not to specifically refer to H. zosterae. That being said, "a pair" of H. zosterae would not work well in a 12 gallon. They're only about the size of a quarter, and don't do well with any swimming tankmates, so you'd have an awfully empty tank. Depending on how much you modify the tank to prevent them from getting sucked into filtration or blown around, and to bring the temperature down to 71-74 degrees, you might be able to keep several pairs of H. zosterae in there. That being said, I'd do a lot of reading on their care requirements first. They require daily hatching of brine shrimp as a food source and very dedicated care.
As far as other available seahorses that are not H. zosterae, regardless of what your LFS tells you, they will all reach sizes that make them too large for a 12 gallon tank. Seems LFS like to label juveniles of the larger species as "dwarves" to sell them to people with smaller tanks; but dwarf seahorses (H. zosterae) only reach 1", and are extremely tiny and very rarely carried by LFS.
