Blood parrots can get pretty big. I kept two in a 75g for less than a year. I bought them around the 3 inch mark as well and when I sold them, they were each about 8 inches long and 4-5 inches tall with large bioload.
I know you are asking about South americans, but would you consider small central americans? A BP after all is part central american... we think. I'd suggest something along the lines of Firemouths, T. pasiones or HRPs, maybe Cryptoheros cutteri, or spilurus... I wouldn't get anything that gets too large because the BP may see it as a threat, and they can get aggressive. That's why I got rid of mine.