It is an interesting one because to me from an animal welfare perspective there is nothing wrong with short periods of stress where zoo animals are exhibiting some territorial behaviour / intimidation but you need to avoid chronic stressors. I'm not up on the literature any more but there was some compelling evidence in animal behaviour circles when I did my undergrad that never allowing zoo animals to experience fear / discomfort / social aggression is as damaging to them as it would be for a human child. Obviously you have to manage risk but meerkats kill other meerkats in zoos relatively often but we still agree they should be kept with other meerkats. If you minimise the risk and ensure different species have safe spaces they can retreat to then I see no problem with mixed exhibits. But badly designed or as a gimmick for the public it is clearly bad.
It is an uneding question in the industry, but zoos do typically try their hardest to avoid life-threatening situations between animals. Cheetahs and rhinos together is a good example. On the one hand, yes, you might think that a rhino would never see a cheetah as a threat, that a cheetah would never have a go at a rhino, but there are so many variables to consider when putting these two together. Does the cheetah have a place it can go where the rhino can't get to it? Maybe. Are there also places in the habitat where the cheetah can't avoid the rhino?
Do we want to breed either animal? For both animals (any cheetah and any rhino subspecies, even the less endangered southern white rhinoceros) captive breeding is hugely important. So if the rhino has a calf, can we trust the rhino to protect the calf from the cheetah, if the cheetah should ever decide to go after the calf? Maybe, but at what cost to the cheetah? Can we guarantee that the cheetah will never go for the rhino calf? No.
Typically zoos introduce stressors via a very simple method - changing the environment. This is especially effective in primates, as primates typically hate sudden change (just as humans do). I remember when Auckland Zoo used to have chimpanzees and they totally upgraded their habitat with new climbing equipment. They expressely said that the chimps would hate it, that it would freak them out, but that it was important to do it anyway, otherwise the chimps would become bored and complacent. Same with their orangutans (twice now, I think). They also did it with their Sumatran tigers once, adding in some basic logs (transported by their elephants, as a little fun fact - the elephants were stimulated by the scent of a predator as they moved the logs into the tiger enclosure, as well as by the work they were doing). The tigers were terrified of the logs at first. What are these? Where did they come from? We were inside the normal amount of time, but we came out, and here's some stuff that wasn't there before. Soon they were exploring it, marking it, climbing on it.
Point is, unconventional mixed-species habitats aren't a surefire way of achieving good mental health in animals and can have disastrous effects if not managed
extremely carefully. There was even a case I heard about once where a baby giraffe was so fascinated by an ostrich it shared its habitat with that the ostrich was constantly stressed and started plucking.