I just completed act 1 of the Main Quest. I have yet to do the CoW, DB, Companions, Civil War quest lines. Would this be an appropriate time to start them? If so, is there an appropriate time to pause them and start act 2 of the main quest?

It really doesn't matter. Just know that, for the CoW, you need to have slightly more magicka than that which you start with.

Well, the MQ will have only small alterations, but as a whole, it doesn't affect its main story.