A certified teacher will easily get a job in the many language schools in Brazil. The problem is that they pay you a pittance, which will barely be enough for you to rent a one-bedroom apartment in a decent part of the city. Now, if you do have experience and a certificate, you can earn more by teaching adults in private classes.
I've met quite a few of "native speakers" in language schools. They were usually the adventurous types who wanted to see the world and they didn't care about being overworked and earning little, as long as they could enjoy the outdoors year round. Some stayed just one semester before moving to another South American country. Some others end up staying longer. Sometimes they used to share apartments. I liked hanging out with them, as most were delighted with their experiences in teaching, in Rio, going to the beach, knowing a different culture.
I met a 20yo Canadian guy who had to return home after four months, after having been diagnosed with malnutrition. Basically, it was the fist time he was leaving home, and he seriously neglected a healthy diet...
I also met this English woman who had travelled all over Brazil in a bike (!) and was now settling in Rio. I lost contact with her, but she had been here for at least three years.
If you ever come to Rio, I can indicate which language schools offer better environments and pay more.