Actually, I disagree as far as aggression is concerned. Although haremic in nature, in aquaria most flasher and fairy wrasse females will transition into males even with another male present. This, in many cases, leads to the demise of one of the males.
While there may be a degree of aggression towards males of a different species, it is nowhere near as bad as aggression within the same species. With multiple species it usually works itself out relatively quickly and painlessly, especially if a social acclimation box is utilized. Granted, there are some species that are exceptions, but there are a few on the forums that can provide insight.