The breeder has a point there, the species you mentioned will probably not cross with each other, but the australe and scheeli are suspect. The Rivulus magdalenae will clearly not cross with the australe or scheeli but they are pretty big-sized for Rivulus species.
The species assortment isn't very good, because Fundulopanchax species, in this case the scheeli will possibly and most likely bully the australes into submission, and may cause stress to the Rivulus as well.
Riv. magdalenae is a pretty drab fish so its colours will not suit the flashy colour of both scheeli and australe.
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