I usually prefer more of the same species to get the social component of their behavior.
All pygmies and most dwarfs can be kept in harem groups. Some dwarfs live in bonded pairs (C. bicolor, C. eibli, C. narcosis, C. nox, C. venusta).
Especially the pygmies tend to go crazy when kept alone.
Closely related species may form inhomogeneous pairs if they can't find a mate of their own kind (happens even in the wild).