I highly doubt you'll make it, QR1363 have a very odd pattern for time of arrival, sometime even 30 minutes early but just as common 30 minutes late, besides the flights you mentioned (with AF being either a 777-300 or an A380) there's couple more flights which arrive around the same time which are just as likely to cause a long queue at immigration if timings works against you.
For the later flight, George airport is tiny so I think you have every chance of getting to Knysna by 18:00ish.
There is another option though - you could book a meet & greet service, I never tried them but I'm pretty sure they'll allow you to skip any immigration queue, there is still the chance of your QR flight arriving too late for you to be able to drop your bag