Hi ..you should consider to include native auxiliaries both as porters (essential component) and fighters .
From what I’ve read, majority of battles saw also the participation, aside the force publique regulars, also semi-naked tribesmen armed with spear and old ML. Even the porters were sometime armed and participated at some succesful fights.
Against the Dervishes the Force took the field together with Azande tribesmen that had become their allies. I don t game with 15mm but I suppose that Zulu castings without shield or with a different shaped shield (squared) would be fine.
Don’t forget also to provide the columns , in some occasions such as during the Dervish campaign, with a small Krupp mountain artillery piece , dismantled and carried by porters. Small river boats provided with artillery piece or used to land troops were also employed. And finally also in Congo a kind of Rorke Drift battle took place with a Belgian fort attacked by Azande tribesmen. You can use the same small fort again for another historical epic siege that took place in 1914 with Belgian and Rhodesian native police troops besieged by German Askaris from Deutsche Ost Africa Afrika.