I have quite a bit of experience photographing redwood forests, and here's what I've learned:
Use a tripod so you can use slow shutter speeds, since, as you said, it's pretty dark. (I don't have the patience to use a tripod under most conditions, but I've learned that in dark forests I pretty much have to.) Of course, it can be difficult to do that when shooting moving wildlife (something I very rarely do).
Do a great deal of exposure bracketing, as the dynamic range can be very large (dark at ground level, but with bits of bright sky). You can choose the best one once you get them into the raw converter. Sometimes, go ahead and let small bits of sky between trees be blown out, to get the rest bright enough, but make sure you have some darker shots too in case the blown-out sky looks bad; wait until the raw converter to make that decision.
Better yet, because of the high dynamic range, take those multiple exposure-bracketed shots and use HDR.
In those high-dynamic-range situations, see if you can eliminate the sky entirely from the shot, by pointing the camera more down than up, if the parts with the sky aren't crucial to the composition.
Like you, I find that ISO 800-1000 is sometimes enough to get a decent exposure, and doesn't produce too much noise if you're using a DSLR, for situations when using a tripod isn't feasible for some reason.
I haven't tried using a polarizer in conditions that dark, just because it cuts yet more of the light and requires yet longer exposures. Perhaps I should try it to see the difference it makes in reflections. When I've had reflections off the foliage in the past, I've usually just darkened those bits in post-processing.
P.S. Forests are also one of the best places, in my experience, for IR photography! Dark trunks and stems, pale foliage...