Praecox would probably be a good choice. I am using them as dithers in my A. calvus tank, as they do not get terribly large.
The action of the smaller schooling fish swimming around reassures the larger fish that there is no danger. When you have no dithers, some fish will hide continuously, thinking that if there are no other fish swimming around there must be danger. It makes them feel more comfortable to have some schooling going on.