A pacu or two would go well with oscars.....as long as you had a tank of a few thousand gallon capacity.
As stated in a previous post....pacu really get to big for the average home aquarium....2ft for the red pacu and 3ft for the black.
Catfish/plecos on the other hand would do well with most species of african cichlids and dependfing on the species (of cichlid and catfish) would do well with most S/C American species.
Clown loaches, silver dollars, Rainbows would do well with S/C American cichlids as well.....depending on the species of cichlid.
CM