Flowerhorns are best kept alone but they can be kept with larger cichlids that are aggressive, Oscars will most likely be troubled.... Flowerhorns will definitely haress small fish like loaches... As the flowerhorn grows he will start showing his true aggression and may harm fish that are smaller than him..
If you do plan to keep a flowerhorn with other fish, be sure to have a back up plan..
P.S the people who say that they keep flowerhorns in community tanks, are heading towards fish disaster.. the most docile flowerhorn will become aggressive when he realizes that he can bully other fish...