I don't know actually. Having a slaved black population coexisting with white colonizers in brazil is one thing. Having native and hostile black populations neighbouring a portuguese settlement is something entirely different. Mixed race individulas were, in the very early colonial period, half indians and half whites.
If the colonization of the cape is early enough, it's likely to change the portuguese colonial policy regarding blacks.
How so? The Portuguese pretty much pioneered the transatlantic slave trade, with their trial run for profitable sugar plantations with imported African labor being Fernando Po island right next to Cameroon, in the early 16th century.
How much of SA is suitable for sugarcane? (looks at a map) Haha, hahahhaha. Natal. IE. Zululand.
Factors for more slaves:
1.
Easy availability
2. Established practice
3. Sugar profits
4. Don't die like flies (once they get Bantus)
5. Tribe A hates tribe B (low solidarity)
Factors for fewer slaves
1. Euros can multiply there (even Natal outside malaria zone)
2. Sugar zone is far from Cape, not likely to be taken early.
3. Not established practice by 1520? I think.
4. Easy to run at frontier and/or higher slave densities.
5. Local slaves are close to familiar territory- their friends and relatives are right there (if still alive).
1st. Assertion: After initial problems with Khoisan slaves, South Africa is integrated into the worldwide slave trade; mainly exporting Khoisan (get 'em while supplies last!) and Bantu to the Caribbean and Brazil, and importing from Arabia, India, and Malaysia (war captives).
On the other hand, the Trekboers somehow managed massive numbers of black slaves without it causing them too many problems. I guess it helped that the natives would kill them just as hard if they got the chance.
Thinking of the analogy with conditions that caused the North South divide in the US (glances at Tobacco map of SA), Cape province is agriculturally 'north' of the Mason-Dixon line.
2nd Assertion: Slavery of locals in SA will become of greater prevalence after expanding into tobacco and sugar regions where Bantu are thick on the ground. It'll REALLY become prevalent where the Portuguese advance into the disease zone.
3rd assertion: Fernando Po is still happening. The lure of great profits is still there. Portugal is still going to be the world champion of the slave trade for a loooong time, just like OTL, if they still get Brazil or other suitable territory.