Can you give us a photo?

Nothing is going to be able to play with the trees. If you can get the shrubs and ferns out far enough that it will be several years before the trees overshadow them, maybe they have a chance.

They will accommodate one another. That is, they will-each plant-grow exactly as much or as little as allowed for by the competition of other plant types present in the same area. Only you can judge whether or not the effect is suitable. It's an aesthetic judgement call, and we can't make those for you. Oh, we'll try, lol, but in the end, it's up to you.

Ostrich fern-Matteucia struthiopteris-is an aggressive rhizomatous spreader. This is a good thing where coverage and profusion of planting is the goal. I wouldn't plant it in amongst my choice perennials, since it will march from one end of the bed to the other, and then out into the lawn! That said, I've always had a few ostrich ferns around my yard. Great in early summer. Dry years, they fall down by August, but that's acceptable in my scheme of things. My very favorite ostrich fern scene is wet ditches in full sun in northern Wisconsin-chock-full of these ferns. Looks very cool like that, with the sun hitting them. But your thing should work.

The picture captures one section of the boundary. The stockade fence becomes a net fence, behind which are brambles. I need the trees for the screening they provide to the view beyond the brambles, which is a pool on a higher elevation. I had hoped the ostrich ferns would obscure the legs of the trees and the fences, but your comments suggest that they will die for want of sun (although as you see the trees are fairly thin.) If that's the case, is there something else that tolerates more shade? The shrubs have been planted far enough forward that they are doing alright. I know you are right that the aesthetic call on ferns mixed with shrubs of more or less equal height (tbd of course) is one I have to make. But I'm afraid it will be too late if I decide I don't like it. That's why I hoped for some examples..

Ostrich ferns will certainly like shade--if there is enough moisture. And if there is a LOT of moisture, they'll also grow in full sun. However, if they are really happy, they make eventually choke the Pieris and Clethra.

I have a different type of fern that has grown in around some still small shrubs, and the shading they provide to the shrubs has really slowed their growth. I think it is more the competition from tree roots and dryness that will influence how well all the plants grow rather than the shade per se. I can't see the shrubs in your photo, but I would probably try planting the ferns back along the fence and the shrubs about 2' out into the lawn (after removing that strip of grass) to allow everything to get a good start, and then mulch everything well. The ferns will grow outward to mingle with the trees, and the Pieris/Andromeda and Clethra will bloom better and grow more fully with more sun and less root competition from the trees and ferns. The yellow fall color of the Clethra will look stunning against the dark evergreens.