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.