Julidochromis can be a little nastier than Lelupi but if you want to go this route start out very young, Calvus get a lot bigger than most shell dwellers so in the long run your shellies will become food, as for a good starting point you could go with Neolamprologus Multifasciatus (shellie) or Neomprologus Ocellatus (shellie) Just because shellies are small does not mean that they are shy or less aggressive they can really hold there own.. Really a 40 breeder footprint is not that great when it comes to multiple species so if you def want to go the route of adding a couple of species then as stated above you can add one of the shellies that i have mentioned and a rock dweller like the Julidochromis or Lelupi..
Sand for substrate, rocks on one side piled midway to top and a crap load of shells on the other side, as everything progresses you are more than likely not going to have a big colony of shellies due to predating from whatever you decide to keep together.. But if you def start out with small species they can grow up together, shellies mature a whole lot faster and will start breeding and form a strong colony by the time your other species reach maturity..
Good luck with the setup..
