I've found in the past the only way to keep the more aggressive species of Tanganyikan's is with the addition of Cyprichromis. Wether it be species of Callochromis, or Ventralis, Cyp's usually solve all problem's. A 40 gallon is a bit small for the two species to co-exist, so instead I would crowd the group of Callochromis in your 40. Instead of 7, start with a group of 10, 1.5in. juvie's. Down the road you may lose a few, but if all work's well you should be left with a nice group, good luck.