Replace 7" metal duct with 6" flexible or 8" flexible?
You're probably fine with the larger diameter (I wouldn't go smaller), but I would install a damper where you tie into the main trunk and then have someone come and rebalance the system using flow meters. That way a professional can optimize your entire house so that the air entering each room is the same relative to it's size and heat load, and the house will stay more comfortable as a result.