My experience with melanura(and any Vieja for that matter) is that they "do" do well with "dis" similar cichlids, and most non-cichlids, but not well with other Vieja or other high bodied similar pan shaped cichlid species. Of course for me, this was in tanks around 150 gallons, but if the tank is 300 gallons plus, there may be enough room for this not to be a problem, in a 125 maybe not.
I agree about the Thorichthys or sometimes Parachromis being suitable
Whether the chocolate is seen as competition is debatable.
If the Vieja does eventually see the chocolate as competition, the chocolate may end up toast.
It will take years, but the clown loaches alone, will outgrow, and become too large for a 125 gal