Hi David,
You might be interested in the information Sheldon Frith has compiled, here is the thread he recently posted
https://permies.com/t/50032/plants/Ecological-Companion-Planting-Guide-based#402867
I think you'll find that the fungi to bacteria ratio that redwoods prefer is in the 100:1 to 1000:1 range. You could check what you have in yourpasture and maybe do some fungal enhancing if it's not near that. They'll grow before it gets to that ratio, and the trees will move the ratio towards that, but you'll be off to a better start if you can get the fungi in the soil for the trees. This will also give you some ideas what will go well beneath them, maybe strawberries, raspberries, apples, and stone fruits.
About "western Red Cedar" I can be of no help as in the US there are several plants that go by this name. The one in my region is actually a juniper and is adapted to dry conditions. The one in the pacific northwest is botanically not a cedar, and there is incense cedar, one of my favorite trees. Here is a link for incense cedar, which might be fine for your north facing slopes
http://www.nwplants.com/business/catalog/cal_dec.html
About the redwoods, I lived in the redwood forests of northern california for a few years when I was a teenager. They grow with their roots on or in the banks of small streams. When flooding changes the level of the soil, they grow new roots where gravel has buried several feet of their trunks. When I think of the flood cycles in the thousands of years they have lived there, I feel pretty certain that once established they can stand wet feet in the winter just fine. I also think they would survive the occasional very warm day in the summer.
One thing about redwoods, a single one left standing alone when all the others are harvest can get blown over. It is better to plant them in groves or groups where they provide some protection for one another. And when harvesting, to leave groups of them. I think they also will send up new trunks from established roots.
Thekla