I am planting a 50' long flower bed. split in half by a set of front stairs. I need to know how many plants per side. I want to leave room for these to grow. I was told between 18''-24'' between plants. That seems a little high. Doing that only gives me approx 6 plants per side..... any suggestions????? Also, I am putting in Holly shrubs, hosta, astilbe, acontium, barbarry. It is a shady area. do these plants sound good?