Firebrick and Cob

We are building our first RMH, here you see the start of our Brick Heat Riser! It is a very simple process of legos however we have a question about joining the bricks together. We have read that you shouldnt get Firebrick wet? If we use cob (Clay, Sand, Straw) and put this on the firebrick, that would get the firebricks wet. Wouldnt they crack when we burn a fire in it?

You can get the firebrick wet to a certain degree. Using cob in contact with the firebrick won't cause erosion of the firebrick so long as you let the project dry before firing.
If you absolutely must fire the project early, a slow fire would be best. You would be using the heat from the small curing fire to dry all materials. Getting the fire roaring hot while there is still a high level of moisture in the cob would be inadvisable.