Have taken-care for a large number of species and esp. many of the smaller ones ( genus Tockus ) were kept with other larger bird-species ( Touracos, Rollers, Pheasants, Waterfowl, larger corvis-species and so on ).
The Ground hornbills are mixed at many places with ( african ) hoofstock and I've seen them also being kept with vultures and large wading birds ( storks, marabous ).