I vote for adding after you have reached the FG. Many times you lose the flavor of the spices when you add them during fermentation. Just remember that the cinnamon stick will impart a sediment if you leave it intact long enough.
You may want to consider making a cinnamon extract and vanilla extract--simple way to add a nice spice profile with better outcome when compared to using the whole spice. By the time the mead is clear, aged, ready to bottle you will have access to excellent extracts that you made. You will appreciate them so much more than a vanilla bean or cinnamon stick added to a carboy, and you may find yourself gifting little bottles of vanilla to people.
Simply grab a bottle of high proof neutral spirit, vodka or pure grain, and tuck in a few split vanilla beans(I use 5-8 six inch beans per liter) and the same for cinnamon sticks. At least two months(even longer the better) in a dark cabinet, shake it when you think about it; you can strain/coffee filter the cinnamon to get rid of the sediment but no need to do that to vanilla, just leave those beans in for as long as you want. I tend to use five drops of my vanilla extract per 750ml bottle to add a nice vanilla profile, but taste trials are necessary because the objective does vary. Cinnamon is always a taste trial. But the extracts are awesome as you prepare to bottle, dabble in the kitchen, and add to your coffee!