A breeding pair of medium size geos (maxing out at no more than 6-7") would work. Conspecifics is usually low enough that mates killing one another is more rare, but does still happen from time to time.
Why would you say there is no room for dithers? Using small neons or cardinal tetras would work as long as the breeding pair doesn't have large mouths (geos would). You could even do some livebearers (4-6 maybe).
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