Ha, the more I learn about cichlids the more I realize how little I do know. Besides, by knowledge is pretty restricted to SA dwarf cichlids. I wouldn't know a rare mbuna if it swam up my......well anywho......
They will accept anything as a cave, but the females seem to prefer very small spaces with one small hole. I usually just pile up some slate.
what are your water parameters? I've found alot of apistos will breed in average tap water (some more than others), and all the ones I've kept can be maintained in it indefinatley. If you want to spawn them, simply maintain them in tap water while you condition them with lots of bloodworms, mysis, and live bbs (you'll need them for the fry, so might as well get good at hatching them). When the female looks good, do a 50% or larger water change with straight RO water. This will most likely trigger a spawning. Once the babies are a week old, or so, then you can start doing WCs with tap again.
I did not have a whole lot of success with my panduros (similar to nijsseni). They got along great in their 10g, and even spawned a few times with fertile eggs, but they always dissapeared after hatching (usually apistos are excellent parents but this pair was crappy)