Duct work requires math, arbitrarly just putting on pipe is not always a good idea. you need to find someone to size the ducts you need. Not doing this can cause undue stress on a system by either too much back pressure or not enough and the system runs too long. There is a science to this. Consult a local HVAC designer for more assistance.
Actually I have the sizes determined for both the supply and the return.
What I am looking for is how to run lines off one size flex duct (10") to another.
It seems odd to put sheet metal tees in each joint.
What is the best method to branch off or change size in a run?