If the wall is any good the trees won't knock it over. Spruce doesn't have a tap root but that doesn't pertain anyway. The only issue would be if big trees right near the wall might affect it when they are big enough. In Zone 6 you are probably talking about 50 or 100 years before that might come up. Maybe small trees are adequate for your screen anyway.

I think 15 feet sounds like plenty of space between the tree and the wall, so I would go ahead and plant the spruce if you like. I would probably choose hemlock instead, since I like native plants and particularly like hemlock which should grow very well - it is common all over western PA. Deer will be a bit more of a problem with Hemlock than with Spruce, but either should be OK, I think.

Hemlock have shallow roots. I seen a fence made from hemlock. they planted them close together then cut them to about 6 feet so the trees got wider and thick. In our area of PA we have adelgid which is a parasite thats killing the hemlocks.