South American:
An exhibit mimicking that of a treetop rainforest canopy with branches, vines, etc. where guests can watch monkeys playing. In such an exhibit could black howler monkeys be exhibited with white-faced saki, capuchins, Goeldi monkeys, squirrel monkeys, and emperor tamarins? What if the exhibit is taken a step further to include extended flooring with soth American plants, and a swimming area where capybara and Brazillian tapirs can swim (complete with underwater viewing)? Could the two get along with the monkeys, or would the monkeys cause them irritation?
Your first suggestion requires a bit more thinking capability than I have at the moment (it's late in Europe), but I can probably answer this one:
Howlers monkeys, Sakis and Tamarins (including Goeldi's) have all been mixed together before. And capybaras should have zero problems with these animals. Though Lowland tapirs would be a new addition to this mix, I don't see them causing any problems to this exhibit. However, Capuchins and Squirrel monkeys are usually kept separate from Saki's or Tamarins.
If you want it both ways, I would suggest having this exhibit setup:
Exhibit 1 with Black howler monkey, White-faced saki, Emperor tamarin, Goeldi's marmoset and Capybara
Exhibit 2 with Capuchin or Squirrel monkeys and Brazilian tapirs.
For an extra interesting exhibit complex though, you could create a moat in between the two areas which the monkeys cannot get over, but the Capybaras and Tapirs can swim through as they please. This way the semi-aquatic animals can get to both exhibits while keeping the monkeys safely separate (though there might be risk of monkeys hitching a ride on a tapir. Make sure the moat is large and deep enough that the Monkeys won't risk jumping on a tapir head to get to the other side. A bridge or platform between the islands that forces the larger mammals to dive would prevent this best, but it would be difficult to incorporate while keeping it good-looking).
Also: there does exist a thread specifically to ask these sorts of questions