Depends. If fragging from your DT, then yes. Adding a frag system to your current system for this reason is fine. New coral frags should be QT in a separate system tank until cleared for pests. It's really the best way to do this.
It changes from situation to situation. I did it with mine and I love how it’s so stable. The larger volume is what I needed for my tanks to be happier. I was having tons of issues with my tanks that I needed it. sometime it isn’t good and you can wipe out your whole live stock with parisites, diseases, etc.
It depends on why you are adding a frag tank,
if the goal is just to trim down you colony and have an area for the frags to heal before they move on yes.
if the goal is fur culturing purposes I would say no. A separate system to grow some frags into colonies to frag from to cultivate is better. You have a back up system in case one crashes and can continue on, since you did not lose everything.