A lot of zoos that I have visited often have either large mixed paddocks for a variety of bigger South American animals (tapirs, capybara, giant anteater and similar) or a walkthrough mixed enclosure for smaller animals (small primates, birds and xenarthra particularly), so I have mainly chosen species kept in those sorts of displays, with some exceptions:
1. Six banded armadillo: Although not the most common zoo armadillo, they are a fairly large species that is often kept in mixed with primates; they also have the advantage of being the species kept most often in large and well-landscaped outdoor enclosures rather than smaller indoor spaces.
2. Northern helmeted curassow: While any manner of ground birds are often kept in walkthrough jungle enclosures (such as sunbittern, giant wood rail and various ducks), this is a large, endangered species that is also, here in Europe, the most commonly-kept curassow.
3. A smaller monkey: Either the Bolivian squirrel monkey or the white faced saki would be my ideal choices; both species are often kept in mixed enclosures and walkthrough exhibits, certainly here in Europe.
4. Azara's agouti: One of the more commonly-kept species of agouti, which have been mixed successfully with armadillo, the capuchin, howler, saki and squirrel monkeys, giant anteater, capybara and curassow and is kept in walkthrough enclosures.
5. Venezuelan red howler: I think it is the better-looking of the howler monkey species, has been mixed with agouti, curassow, saki and squirrel monkey (other species of howler have been successfully mixed with six banded armadillo, capybara and spectacled bear) and are housed in walkthrough exhibits in at least two zoos.
6. Maned wolf: One of the most visually-distinct canids still missing from the game that is increasingly turning up in smaller zoos as well as larger ones - they have been successfully mixed with a variety of species, most notably the giant anteater, although I am not sure I would give them a mixed-species bonus.
7. Spectacled bear: One of the most unique bears still yet to be put in the game and my personal choice for a new bear; they have been mixed with other species although, as with the wolf, I would hesitate to give any species a bonus with them.
Exhibit animal. Not so fussed on the exhibit species, as South America already has more exhibit animals than Africa, Europe and Asia combined - maybe an insect (one that actually exists in captivity, unlike the titan beetle) such as the black spot mantis, the giant violet-winged grasshopper or black beauty stick insect.