This post, and the talk about gudeons, reminds me of the sparkling gourami a previous poster had. Ideally a small group or trio of one male with two females, but one female will be dominant, and sometimes decide to brutalise the more submissive female, and take shreds out of the male from time to time, even while they were breeding. I'll try and see if I can find her threads about it, could be useful for OP.
Eventually, she needed to remove the submissive female since she was being beaten down and chased too much. All the advice was to have a small group, but they seem to have a pretty complex hierarchy and defined territories, that getting the right balance enough space and hardscape/plants to allow them to create defined territories - and as you said, introducing them in the right way while carefully observing to monitor and rescue where needed... sound as though they're similar fish to these gudeons, and your apistos.