Plecís should be able to handle their own.... they are basically metal plated
usually plecís hide out in one spot anyway....
I canít comment on if a terror will get along with your bully Oscar, as far as I know oscars are so personable they can vary far left and far right in terms of suitable tank makes.... try them together if you can, but keep an eye on them.
Correct me if Iím wrong but to properly distribute aggression we would have to ďoverstockĒ the tank? Iím not sully up to speed with South American cichlids so.... take it with a grain of salt