In my 75 I have 4 HRP (3 platinum and 1 regular) and 2 Firemouths. They do well together and the HRPs just had fry last week that I now have separated. HRP's I think will do well with most fish the same size. They aren't overly aggressive. Even when breeding they aren't too bad not nearly as bad as the breeding firemouths that I had.
I have an electric yellow lab that is much smaller than the HRP if it shows any aggression it will be gone but so far it keeps to him self.
In the tank is also 2 green phantom plecos and a bulldog pleco.
Also don't over stock the tank. I had 6 HRP's and 4 FM when I started and they are showing much more personality now that there is less in the tank. They come to the tank and watch me instead of battling each other all the time. when they see the food they wag there bodies like a dog.