Are living style guides worth the effort is small projects?
I think documentation is always helpful, as from the get go it helps you organize your designs and code. Now the value of a full fledge living style guide will depend on the size and length of the project. I've worked it 3 month projects where a lot of development was done, and having a living style guide was super valuable. But in smaller projects, that only last weeks (which are typically proofs of concept) it can be overkill. In that case, I would still build in a modular way and add documentation, because if the app needs to scale, it will be very easy to plug it into a style guide generator and from there create a living style guide.
Another angle to this is if you are using over an over the same library of components as a baseline (think of Bootstrap or your own home-brewed framework), then pouring over documentation should be pretty easy and will allow you to quickly jumpstart projects, even if these are for a short term period.