If I’m going for the grainy retro look for some lowlight photos would it make sense to keep ISO low and add grain in post, or would the grain from turning up the ISO give a different kind of grainy look

“Grain” is a property of film, not digital. If you want to have a “film” like grain, the best option is to add it in processing or post-processing stage in computer - you have infinite options then to define what kind of uglyness you want.

Generally one should capture as much information as possible - it’s always easy to make low quality (for example the retro look you’re after) from high quality information, than try to get a low quality shot right away on the camera and hope it’s the right kind of low quality ;)

To collect lots of information, you should collect as much light as possible. Thus go for as long exposure and as large aperture as possible (and then you can set the ISO to as large as possible without burning anything, though often it’s much more practical just to let the ISO float automatically.)