The polarizer will vary depending on its orientation. I have had good luck metering through the polarizer when using a camera with TTL metering. I have not tried one without, so I can't answer the question about specific factors. With a red filter I do not trust the meter. The TTL meter on my camera will tend to show about 2 stops and I adjust by another stop to make sure. As for the differences between films, they do have differing levels of red sensitivity, so I would be inclined to follow the tech specs, I have just never looked it up myself.

As for the additive nature of the filters, logic tells me that you just add the numbers of stops of correction. If you are using a wide angle lens, you should have plenty of vignetting, depending on the lens. I get vignetting with my Mamiya 45mm 645 lens with two flters, three would be a huge amount.