Fall is the best time for planting buy then. The hollowed crown could be from many things but most older peonies when divided have a woody portion that is sometimes hollowed out. Cut out, you might have to use a small saw the hollow places if posible. Allow to air dry to prevent mold and fungi from entering the root before planting.

Think of what you want to have. Two feet apart will form a hedge after a time. Good air circulation will prevent many problems from occuring.

Don't worry about the hollow spots, but do think about the spacing as at two feet you well could be creating a mildew problem in the years to come. Al