Hi,
Checking by tag is a good idea and simple enough. I would go down that route happily myself.
when you'll get more expert with PlayMaker, you'll find you could have your shape prefabs featuring an fsm hosting variables defining the shape, and from other fsm you could query them and find out everything about the shape being selected, dropped on or dragged.
you'll find that with tag you can only have 1 information, in your descriptiong this is fine, but what if you start to have more complex game logic, for example, a shape and a color, then tag won't be enough or at last become a bit clumsy to work with.
So go for tags right now, and be aware that along the way you may need to refactor this and implement something a bit more rich if more properties is needed to define a particular shape.
Bye,
Jean