2x6 24" OC is common practice in stick framing for 1 story structures. 24" provides adequate sheet rock and plywood backing while keeping heat transfer to a minimum.
I have never encountered a 12” spacing for lateral. Usually these problems can be solved with a tighter nailing schedule on the same centers and/or thicker sheathing.
If I were enclosing a frame with stick building I would want to avoid redundant framing, specifically load bearing elements. I would include some typical frame details showing openings with minimal framing (light headers, no trimmers) in the constuction docs so as to get approval from the plan checkers before building and (theoreticaly)avoid debates with the building inspector.
These details could be worked out with the engineer whom you have probably hired to calc the frame. If the stick frame is to impart lateral to the timber frame then the engineer would have input on that attachment as well.
