Pencil fish all make good dithers, the only reason for not suggesting N. beckfordii is that it is a much more efficient fry hunter than the others. The Hockey-stick Pencil (N. eques) is absolutely ideal as it has a little mouth.Why not pencil fish?
This is true in my experience, I've never kept Rummynoses, but all the other shoaling fish I've had have tended to disperse through the tank, and only group up if disturbed.The vast majority of so-called shoaling fish soon settle into their new home, and consequently stop shoaling. The shoaling is a defence mechanism from predators. Once the fish learn that there is no predators, then they usually swim about 'randomly'. Rummynose are often the exception here and seem to shoal tightly almost all of the time - especiallly in larger aquariums.
I like Ember Tetras as well, and the little Rasbora spp. look great and would all work, but I think they might all end up in the lower greenery, rather than visible near the top of the tank.I really like Ember tetras (Hyphessobrycon amandae). They stay small and in appropriate water colour up beautifully.