Ignoring the site of SCJ, did you check the functional movement of shoulder?
Extention. flextion, abduction, adduction, external and internal rotation by active and passive? did you find any limited motion? If those movement is limited(dysfunctional) then you need to treat those structures before strength exercise.
Treating those structures can reduce loads to shouder and SC joints. make sure you put patients back on the wall when doing abduction adduction, external and internal rotation. patients will cheat movements by doing uncalled moveoument.(anterior superior shoulder movement)