Currencies available to CBSubs are configured within CBSubs > Currencies. If ZAR isn't there you will need to add it, but I see it there on my install so you shouldn't have any issues setting it as the primary, secondary, or as a selectable currency in CBSubs > Settings > Global under "Currency and Price display". To limit a gateway to a specific currency simply edit the gateway and adjust "Accepted Currencies for payment" as needed. Note these parameters use the full currency name so for ZAR you need to select "South African Rand".