A 300 gr solid from a .375 is hard to beat.
I would not use buck shot of any form on the smaller animals like dik-dik, blues, or suni. The chance of only wounding would be very high (think of trying to shoot a jack rabbit with buck shot). If using a shotgun for those three, I can't imagine anything much more effective than a 1 1/8 or 1 1/4 ounce load of No. 4 shot. Though again, a solid is hard to beat.
The other Southern African tiny ten really need to be hunted with a rifle. You may get a shot at a red or common at 30 yards, but it is just as likely to be 130. Oribi tend to also be an open savannah game animal. A shotgun would be a handicap for any of them.