Slate will not create the nooks and crannies when stacked. Round rocks with 40 pounders on the bottom create great cracks/crevices when stacked. As mentioned, the fish like really tight spaces.
Dang, did not think fry would be anything for months... she’s definitely holding then...I had to go back through your posts to see which fish you actually bought.
If you are asking about the Paralabidochromis sp. 'red fin piebald' they are a maternal mouthbrooder. The female will drop an egg, the male will fertilize, the female will pick up the egg then repeat the process until they are done.
What was the other species you bought?