I am at the point in the main quest where I am to arrange a peace treaty between the two sides. I haven't even started the civil war storyline by picking a side. Can I proceed with the peace treaty and the main quest and do the civil war storyline later, or do I have to start and finish the civil war quest line before proceeding with the main quest or else not be able to do the civil war at all?

You can do the peacemeeting and do the civil war later, with no restrictions on which side you can join.

Cool. Thanks for the quick response. I was afraid I might lock myself out of the civil war if I proceeded with the main quest first.

No problem. I happened to peek into this subforum just as you posed the question, so why not give a quick answer?

Yeah, it's not really a peace treaty so much as a truce, i.e. a temporary cessation of hostilities while you deal with the slightly more pressing matter.