I have always run my forges from bottled gas supplies through a propane rated hose. Now I am looking at permanent installation in my shop with a 1000 gallon tank outside. It is currently plumbed for the hvac only. I am going to need to run a branch line to run the forges, either copper or iron pipe from what I can tell code is indifferent. What I am a bit fuzzy on is the connection between the hard line and forge. Am I ok using the rubber hose, or is there something better? ( more legal) I se absolutely no mention of hose at all in the code aside from the metal flex hose used for residential appliances. 

Understand that residential gas appliances run on low pressure (4 oz vs 5-20 psi). So either soft copper or rated rubber are your choices, as flex is not intended for high pressure use. 

Now to the run from the tank and for distribution black pipe is less expensive than copper

I am thinking iron pipe outside, parallel to  the low pressure, with a branch through the wall where the forges will be. It should T off the line before the low pressure, and inside the wall have a shut off and regulator for each branch, then the forge hoses will tie into the regulator on the inside. Am I missing anything?

You're right on track.