I love side planks. They provide an excellent way to stabilize your trunk in all three planes of motion, while lighting up your obliques at the same time. The issue I've come across with the side plank, however, is that most people can't properly stabilize on the elbow that's maintaining contact with the floor.
For client's with problematic shoulders, I tend to stay away from the side plank until we start to resolve the issues present. One regression I use is the off bench oblique hold. By removing the upper extremities from the equation, it's much easier for individuals to stabilize their trunk properly while feeling a kickass contraction.
I'll typically prescribe this drill for anywhere between 5 and 8 breaths. Give it a shot and let me know what you think!