Hello,

I am in the redwoods in the Santa Cruz mountains area, so my personal experience is a little further south...

It seems to be very difficult to keep sequioa sempervirens small, particularly if it is a specimen tree. They tend to grow even faster when there is less competition, and will shoot up very quickly. Topping makes them look very funny!

It is not true that nothing will grow underneath, but it is true that the thing that will grow best underneath are more redwoods. They don't like to be alone, and each tree will send up dozens of new sprouts from its own root system, which can't be pulled like seedlings because they are part of the tree. If you cut them, they will come back quickly, so you just have to accept that the tree will always have a halo of green sprouts around its base. This drives some folks crazy, I don't really mind them. Rhododendron Macrophyllum is used to growing under them, and will get to 10 feet eventually.

Mostly Natives has some redwood cultivars that might be better for you...take a look at sequoia sempervirens 'aptos blue' and 'soquel', which both top out around 60', and are more suitable to your space.

Hope that helps!
Redwoodrose

Here is a link that might be useful: mostly natives nursery, marin