That stag looks great. The twin-stags on cypress you mentioned have really started to pop, and the ones mounted on cork look the same. I only have a handful of stags still in their pots - time to order more "baby" staghorns!
When the fronds get around 6" long, I mount them all, so those potted ones are mostly ready to mount. It has been so doggone cold here, I just haven't been in the mood to do any mounting.
I personally have never had any problems with fungi on or around my staghorns, but that may be only luck, not anything I do special. Cinnamon would certainly be safe enough.