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Two or three pairs of kribs (the smaller species) could be kept in a tank that size, so long as there is sufficient cover for the non-breeding pairs to avoid the dominant pair. Larger kribs, such as P. humilis, may tolerate other larger species in that space, but if they are mixed with smaller species the little guys might get killed.
Any medium sized tetra, barb or danio, as well as livebearers that can handle softer water, are good dithers.
Why not just add a larger group of the same species and see some more authentic interactions between pairs? Your breeding success would probably go through the roof too as it seems to stimulate successful reproduction according to some sources.