So I'm nearly finished with my forge (up reading from a brake drum forge) and I was wondering what I need in terms of stove pipe. Ive already got two pieces of galvanized stove pipe, but I'm concerned about the galvanized with the heat of the forge. Should I get some black stove pipe? And if so how much of the chimney should be black stove pipe vs galvanized assuming the chimney will be 10 ft tall?

I have been using commercial grade 10"  spiral ducting, no problem, typical stove pipe is not big enough for a forge.. Coal smoke , as far as I know, will not cause the zinc to become airborne in the pipe system, I would suggest minimum 10 inch pipe. 3 feet above the roof, 2 feet above anything within 10 fet horizontal.

use the galvi on the outside of the shop and black pipe, or something non-coated inside the shop. The further away from the fire, the cooler the stack will be.

I just added about 16" of sheet steel to the bottom of my 10'x10" spirial seamed duct (US$4 at the habitat for humanity store sale)  I also made the sheet steel part so I can pull it up on the pipe leaving a free area all over the forge for odd shaped projects---Though I have done some forge welds with one piece stuck up the pipe while the rest of the object is buried in the coal pile.  (forge welded legs on a spider)