Generally with trees and shrubs it is best to plant in existing soil and mulch afterward. Special amended beds are more useful to small plants like annual flowers and vegetables with tiny root systems and an genetic background involving popping up in recently disturbed soils with high organic and nutrient content.

Your purpleleaf plum bushes are blooming from overwintering flower buds set the previous year. However with this particular type hard pruning each year right after flowering is recommended. In my climate it may be pretty much mandatory due to disease problems.

Exclude your little garden helpers with effective fencing.

Fertilize if a soil test indicates a need.

Only way to keep the rabbits away from it is to use some chicken wire. Once the bush gets tall enough they will leave it alone because the stems become hard & the leaves will move higher up where they won't reach them. It does get pruned right after flowering & then it sets buds for next spring flowering. I always fertilize it right after blooming is done & that is all it needs. This bush will grow really fast or it has for me. You do know that beetles love this plant & you will fight them off to keep the leaves from being destroyed.