N. caudopunctatus will breed in shells if they can. But they are also fine with caves, flower pots, the aquarium glass or any other smooth surface. N. multifasciatus is way more prone to breed in shells though it will also breed in other structures if no shells are availiable. Normaly the multifasciatus will outcompete the caudopunctatus when it comes to occupying shells.
Yes, calcvus prefer taller openings.
It may not be suitable for a true biotope tank, but large sea snail shells have proven to be good caves for calvus. And in my opinion they still look nice in a lake Tanganyika setup.
The openings shown are tall enough for females but thats all about them. I don't think they will work because they don't dont provide proper cover from the back and they are not on the ground. My calvus never used any elevated caves.
And they are not clearly separated from other caves. Most cichlids living in the rock pile will just consider them as part of their territory. In your case the chances are high that most of the rocks will be Julidochromis territory leaving no space for calvus or other rock dwellers. That is one of the reasons I would prefer the caudopunctatus because they will just occupy whats left. Or another option would be to go only with 2 species.