Constitutionally speaking, a president of the country cannot have a criminal record. Unlike in America, we in SA don't vote for a president, we vote for a party. We then give right to the majority party in parliament to select one of it's MP's as president. To be an MP you cannot have a criminal record hence the big fight against the Scorpions and Travelgate, their gravy train would have come to a big halt.
Mandela and the rest were pardoned and their "terrorist" records were removed hence them being able to become MP's.
So can Zuma become Pres? Technically yes, they can name Motlanthe as their candidate for Pres. Once he become Pres. after the election and if Zuma is convicted, Motlanthe could just pardon Zuma, appoint him as an MP resign from his post as Pres. and have parliament vote for a new President. If the majority is the ANC they could just put Zuma there.
Very unlikely but very possible.
Mbeki's case, however, cannot be changed. He wasn't fired per se, just recalled and therefore he can just clear his name and get some sympathy but that is it for him.