The metal flange/collar will work just fine with residential which is more than most have. High rises and commercial need the firewalls and traps.
However, you shouldn't tee off of another line. It should have its own home run from the main ductwork. You also need a return coming from your new space or not much air will go through the duct anyway.
Also the flex duct is more restrictive than galvanized pipe so you may want to go to a bigger size.
I think you need to have an HVAC company evaluate your needs. The air handler on your furnace/a/c is set up for certain cfm (cubic feet per minute) outputs and the main ducts and branches are designed to handle that flow. The unit is also sized for a certain heat loss in the house and may not handle your new addition which would then affect the rest of the house.
HVAC, like plumbing and electrical, etc. is one of those things that should be designed and engineered before any wood construction starts. You may need a whole new furnace/a/c or another unit put in your addition. Good luck.