I always liked using a few Geophagus,there not as skiddish as silver dollars and they clean the gravel of scraps of food..
I think you have the term 'dither' confused with something else..dither fish are used to draw out shier specie, which in this case, would not be the correct terminology. Target fish, are typically quick, hardy fish that can survive the wrath of breeding pairs or larger fish. Generally the terms are used interchangeably so it's not a big deal IMO...
However, Geos would NOT be a good idea for this tank. They are large fish who are far too timid to withstand an oscar.