Hmm, thankyou for correcting me. I have never bred banggai cardinals. Most fish that keep their larvae in their mouths are called pelagic spawners. However, since the banggai releases the fry as little tiny representations of himself, it is not a pelagic spawner.
You may have some problems with the male eating the fry, so you can remove the male a day ahead of time and remove the eggs manually.
Banggai Cardinals usually take refuge in the long spines of sea urchins. If you can rig up with some dowels and a lump of epoxy a version of the long spine sea urchin, they will feel safer and have a better chance of mating. Also, if you introduce that same rig to the fry, they will immediately take refuge in it as well and have increased survival rates. (less stress).
One thing about breeding is that you have to sometimes cull some of the young. Watch out for signs of missing eyes, poorly developed spines, missing fins, and other abnormalities.