Whether or not hyposalinity may be beneficial for seahorse fry depends on whether they are pelagic or benthic in nature, which varies depending on the species. Seahorse fry that undergo an extended pelagic phase, during which they drift freely with the plankton, are much more troublesome to raise than benthic seahorse fry, which orient to the substrate and seek out hitching posts straightaway. The pelagic fry are difficult because the surface huggers tend to gulp air and suffer fatal buoyancy problems, and may even become entrapped by surface tension. As a result, most hobbyists find that mortality is very high during the pelagic phase. Pelagic seahorse fry may benefit from lower than normal salinity (around 1.016) because it reduces their buoyancy and helps to keep them away from the surface.
Benthic seahorse fry, on the other hand, are bottom huggers and do not need reduced salinity in their nursery tanks. Zulus (Hippocampus capensis) and dwarf seahorses or Pixies (Hippocampus zosterae) produce benthic babies which do just fine at normal salinity. Since you are considering keeping Zulu-lulus (H. capensis), you won’t need to adjust the salinity at all in order to raise the babies.
Best of luck with your seahorses, Nigel!