I've brought several Ruby Red dragonettes, including pairs, through QT, although I have used TTM and not copper or CP. I do preventative treat with metro and kanaplex, as well as prazi.
Some things I've done: Check their bellies. If they are extremely sunken in, especially if you see the hard ridge of their ribs, stay away and get another if you can.
I've used live brine and tiger pods, but I've also had good luck with this product:
https://www.ebay.com/itm/ZU-Feast-R...ankton-Eggs-Feed-Coral-Reef-Tank/162230385838
Putting a little coarse sand or small rubble on the bottom helps the pods to hide and seems to encourage hunting and feeding. I've also kept a little chaeto in QT from my sump as well, after TTM. I've never had them eat frozen.
If they just don't want to eat, it may be necessary to get them in your DT early if you are willing to take the chance on a shortened QT.
Good luck - they are great fish to watch!