I would like to know if there are cadet programs available for low hour pilots in South America.
No, not any more.
If you have a MERCOSUR passport are you allowed to work in all associated countries as a pilot?
No, usually you need to be a citizen of the country you intend to fly in. Different countries have different rules about this so you need to check with the local authority where you intend to fly. The original issuing country of your passport will also play a part in this, not all MERCOSUR passports are created equal, as I'm sure you already know.
TTFN