First, we can't tell if your soil is "too acidic." It isn't likely, but do a pH test to be sure.

I wouldn't backfill with store-bought topsoil. Buy some good compost and mix it with your native soil, about 25% compost and 75% native.

You can plant them now, but I wouldn't put all 25 of them in that trench. They'll be too close together in just a couple of years, and you won't have any to substitute in for failed plants. Plant some in the trench, at least 3-4 feet apart, and plant the rest somewhere else, so you can move them in to fill in for dead plants.

"A few hours of sunlight I hope" isn't enough for this plant in your zone. It will grow adequately in part shade in warmer zones, but you are at the bottom of its hardiness zone range. It will probably not achieve the heights and upright stature it's known for. It may not flower, and may flop over.

Using ornamental grasses as a hedge or wall is not really practical in your zone. The taller warm season grasses need to be cut down to the ground every year in late winter to early spring, and they don't grow back to full height until mid-summer. You'll only get half a year of hedge/wall at best, and that's using a grass which is appropriate.