My only pre-caution to you would be to not plant to close to the spruce. Many evergreen drop their bottom needles if they are shaded out.

There are no magic numbers or maximum height rules, it all depends on the what else it there, the size and shape of the entire planting, and the sizes and shapes of the rest of the plants.

You don't say what kind of spruce, whether they are dwarf or large sizes. 25 large Spruces suggests a fairly large windbreak planting if you spaced them to allow for their mature size without too much overcrowding. In this case, I would say that using other plants in multiples of 3, 5 or 7, or more depending on how large they will get, will be necessary to have any impact. The smaller the individual shrub, the greater the quantity you should use, in general. I have Deutzia gracilis 'Nikko' in zigzagged groupings of eleven in one area and 13 in another. Oakleaf Hydrangea 5 in one area, 3 in another. Rhododendrom maximum, just one as the center of a planting with other shrubs.

Plot the mature sizes of the spruces, and leave enough space between them and the new additions. It will look nekkid for a while, but with good care, will eventually fill in and look much better than if you overcrowded them early on. This applies if you used dwarf plants as well, just scale back your additional plantings in number and size to blend with the scale of the existing plantings when it matures a bit.

Thanks for the advice. Sorry for the omission. I guess I figured everyone would use standard size spruces in a windbreak. I have Serbian, Blue, Norway, Black and White spruce trees grouped together by same type.

One of the more lacking elements in my yard for some reason is autumn color. I inherited the shade trees which turn a pretty boring yellow-brown (oak, ash, silver maple). We have 9 acres of woods across the street and those autumn colors (despite many red maple saplings growing rampantly) are boring too! SO, I'd like to put in some nice shrub color zingers that will make up for that.

I started with some small shrubs but only have two of each-- aronia and clethra-- but I think this dry year may affect their potential color this time.

I figured they were large spruce, but didn't want to assume. You comment about planting additional shrubs on the windward side made me wonder.

Check out any of the Viburnums, Aronia, Vacciniums, Fothergilla, and if you have the space, maybe one of the small trees like Amelanchier for fall color.

Use more fine-textured (small-leaved) shrubs than those with big leaves that make them coarse (lumpy) looking. Plant in drifts, for creation of movement. Try to come up with evergreen shrubs to make framework (bookends) at each end, even if bulk of rest of it is deciduous. Ideally front and middle of main, central part would be planted in bulbs, perennials and annuals but this may be more involved than you want. Plantings of shrubs only look unfinished but adding herbaceous plants jacks the thought and upkeep required up more than a few notches.