ember tetras
i just picked up some Ember Tetras, they are on the current availablity list for PetSmart so if you ask about it they could order them.
They are tiny, a lot smaller than Harlequin Rasboras and are supposed to be reddish, sort of like cherry barbs in some photos, although in the store they were very pale. I figure they will color up in the plant tank the way the serpae tetras did.
Each is about 1/3 the mass of a harlequin I'd say, so you could add a batch of a dozen and hardly make a dent in the bioload. They are smaller than the ghost shrimp. So far they are schooling together, but that could change as they get adjusted.