A CPL should be 2 stops on average, and if it's sky contrast you want, it'll get you that. The only drawback though is that it will (depending on adjustment) probably erase all the specular highlights, the result of which is that skin can look flat and plasticky or over-madeup. I would not expect any vignetting from two stacked filters on a 135mm lens on 35mm, as long as you don't use a step-down ring! And even then, it'll probably be mostly equivalent to using a smaller aperture.

Shooting one stop over (EI100 with the yellow) will be no problem for the film at all. Plenty of people shoot TX at 200 and deliberately so.