Why would the Geo become an issue?
One of the reasons a braziliensus can be problematic is, as it matures, it becomes exceedingly more and more territorial (especially to other cichlids in a confined space like a tank) and the Oscar is not equip to deal with its innate aggression and attitude, oscars are pussy cats in comparison.
Beyond that, braziliensus comes the southern Brazil, Uruguay, and Argentina, thousands of miles south of the Amazon region that oscars inhabit, these areas are subtropical, and even temperate in comparison to the oscars tropical region. If kept at the constant temps oscars like, it tends to make the braziliensus more aggressive.
One other thing to consider is water parameter content.
Oscars prefer very low hardness, neutral to low pH waters.
Braziliensus come from more highly alkaline, harder, water types, they are even found in brackish water.
A different species of geo (such as the Tapajós) mentioned above would be more apropos.
Even young braziliensus types can be quite feisty, and not afraid to challenge cichlids twice or three times their size.

One other quirk about oscars, when their stomach contents are analyzed in nature, some of the most common items found, are Cory's, and other small catfish, beside the normal crustaceans like crawfish, or insects.