Is it possible to style or apply an existing effect to an existing shape to make it "spikey"
For instance given an animal drawing made out of regular shapes such as circles and rounded rectangles all joined into a single path, would I be able to create "spikes" to imitate fur on the outside of the path or would it be a case of just adding triangles where I need to the path one by one?
Would some kind of custom stroke work?
You could use the Crystallize Tool. It can distort a line to add hair-like spikes. Make sure you keep the centre cross-hair of the tool just inside the shape so that the spikes point outwards, rather than inwards.
There are also other tools within this tool group that you might want to experiment with.
Note that you can also double click on the tool to bring up the tool options to refine the way the tool works.
Try selecting your shape and going to Main Menu → Effect → Stylize → Scribble and play with the settings there, also try the predefined values in the dropdown at the top of the panel.
