I have no direct experience with the SAA ERJ's (only the new A320's and larger). My experiences with SAA are that the gate attendants are quite aggressive on the quantity of bags brought on if in economy.
They don't tend to accept the rollie + laptop bag combination that is normal in the European + North American circuit. I have been asked to gate check on more than one occasion. I usually just sneak the bag on once I am around the corner.
I've seen others on this board with similar issues, despite what the guidelines are on the SAA website.