It's not the acid condition that is of primary concern but rather the density of the shade created by the evergreens and their far more aggressive root systems hogging all the soil moisture and nutrients.

There are far more plants that will grow under these conditions than you may think. Since I have no idea where you are located or what may be hardy for you, you will need to do some research on plants for dry shade suited for your area. Some of the classic flowering plants for this situation are fernleaf bleeding heart, bigroot hardy geranium and lamium but there are dozens more, depending on location.

There are a few other issues to keep in mind. It is best to start with small plants to begin with, as it is often difficult to dig a sufficiently generous planting hole in very rooty soil. And the plants will need to be babied for a couple of seasons with extra fertilizer and water until they are established enough to hold their own with the larger conifers. And few plants bloom heavily in heavy shade - impatiens typically give the best show for summer-long color but will need regular water and fertilizer in this situation.

I live in a suburb outside of Chicago, IL. I have tried ferns as well. Yes these evergreens are very old and have a heavy root system.