Since the LA Zoo has exhibited Okapi, they have always been mixed with Black Duikers successfully. And the Red River Hogs shared a yard with Bongos for many years.
It depends on the indivduals, if a mix between okapi and RRH will work. Some okapi can be to nervous to keep them together with other animals, and altough okapi/duiker mix is working very well in most zoos, I know some cases, okapi have hunted small duiker species, so they had to put out the duikers. Personally, I would not keep okapi together with RED River Hogs, because they can be to aggressive, I know RRH have attacked big hoofed animals, bitten and hurt them. Also RRH are active all the day with just short inactive periods. Okapi ar not active that way, so I would not mix them with RRH.
Which zoo is this? If it's in southern California, LA already have experience of mixing okapi with other species and I can't see San Diego asking a bunch of us for advice on species introductions... Sorry if I sound cynical.
I would also suggest not mixing the RRH's. Working around our Okapi I know how temperamental they can be. Duikers historically work well, but our okapi have always disliked smaller animals so they are never mixed together.
Two thoughts: 1. A great deal will depend on the specifics of the enclosure. It's not simply a matter of whether a species mix will work in theory, but in what circumstances has it worked or not worked. This in addition to the vagaries of individual animals' personalities' 2. Strikes me as odd to ask a zoo fan site about curatorial matters rather than asking other keepers and curators. Not that ZooChat lacks some experienced people, but there are better groups for these sorts of questions. I sure hope your inquiries began with colleagues