Welcome to Permies!
Now to
answer your questions as best as I can! I think it really depends on how steep your slope is, as well as just how tall your
trees are and how close they are to the area you're planning on growing. My property is a north-facing slope (some areas more steep than others), with trees in all directions. When I joined permies, I asked the same question as you:
https://permies.com/t/33637/cascadia/Steep-North-Facing-Slope. I haven't really tried doing to much on the steeper portion of my
land, but I've noticed that in the summer I do get quite a good bit of light as the sun is higher in the horizon. In the summer, the sun rises above the trees at about 8:00am, and sets at about 6:00pm, and this gives me a good 12 hours of sun during. During the winter, however, the sun doesn't get above the trees until 10, and sets behind them at around 2:00. At most, I get four hours of sun.
It's important to try to find where your sunniest areas are, and maximize those for things like fruit trees and vegetables that require more sun. A tall
hugel can also make a difference in raising the veggies up above some shadows to get more sun longer. It also helps warms thing up sooner.
As for things growing on my slope, I've had a lot of success with peas, daikon radishes, blackberries, thimbleberries, raspberries, blueberries, wild strawberries, and huckleberries. Elderberries, lingonberries, and serviceberries
should do well; mine just are not mature
enough yet. My
apple trees and peach trees produced, too. And my cherry trees--though probably too young to fruit--did make flowers and look healthy. I'm also pretty new to
gardening, so the fact that my green beans, squash, zucchini, carrots and beats didn't grow that big may be due to a whole lot of factors aside from my slope.
Supposedly north-facing slopes are good for plants that like to fruit early and then have their blossoms die due to a late frost. Compared to my parent's place that's flat and a few miles away, everything on my property blooms a week or two after they do at my parents, since the sunlight is less and the trees aren't convinced to come out of dormancy as soon.
So, questions to ask yourself are: Where are my sunniest spots? How much sun does a place get in the summer compared to the winter? Do your surrounding trees lose their leaves in the winter or not (mine mostly do not)? If most of your trees are evergreen, I think you can pretty much determine your sunniest summer spots by what gets the most sun in the winter. Also, watch during the winter to see where there is frost and where the frost/snow melts soonest. If an area melts sooner than others, it might be a good candidate for your garden.
Here's another
thread with ideas about what to put on a shady slope:
https://permies.com/t/47182/forest-garden/Rick-Food-Plants-Shady-Forest. Some of the plants mentioned are salal, Oregon grape, ramps, ginseng, goldenseal, and
mushrooms.