If you want it as a frag tank, hook it into the DT. This way you only need to dose one place and you can transfer corals back/forth between tanks easily as you figure stuff out.
If you want it as a QT tank, don't hook it into the DT. You will use less chemicals IF you are only treating the frag tank. However, at this stage in the game, if you are getting coral cooties, they are probably in your DT already.
Frag tank/QT tank- 2 different things.
Nothing says a tank has to have a sump. If you decide later that you want to QT, you can always disconnect the return to the main sump and put a heater/powerhead in the tank itself to run for a while.