Given point #3, the N. leleupi are just too aggressive for your aims. That said, one Julidochromis species plus a nice school of Cyprichromis should work just fine. If you chose one of the smaller Julies, like J. ornatus or J. transcriptus, you could also add a shell dweller. I'd avoid L. ocellatus, as they can get pretty aggressive, but other than that pretty much any shellie would work. I'd go with N. brevis myself, but that's just personal preference. Another nice option in lieu of the shellies would be N. caudopunctatus. Good luck!