it all goes down to the temperament of the Clown and/or the Royal...if they're gentle, no harm in it, if, on the contrary, they're "mean " ( clowns more than Royals imo ) your african won't stand a chance....
P. Afer can attain 18" but the most I've heard for X. Nigri is 12" If they seem to be getting along then that's good. I'd keep an eye on all of them though...especially the P. Afer. They can be more territorial than the other knives.