I wouldnt treat all those fish to be honest. I have tried the ole "ichless" tank thing and it is impossible. The ich can hichike right into the tank via a snail/hermit shell or on a piece of coral.
With that being said, while in quarantine he was all alone and feeling comfortable therefore he was resisting the ich. However, when you moved him to DT with other fish that have already grown somewhat territorial his stress meter went up which allowed the ich to attatch.
From my experience, I havnt had a fish die from ich, they died from stress/not eating. If the fish has been eating great and he gets ich while in DT I would just leave him. He will soon establish himself in your tank becoming comfortable and eventually warding off the ich himself. This however wont work if he is stressed out about something wrong with water (ammonia, wierd pH, etc) until it gets fixed. If I have a fish that has ich and is eating heartily, I dont worry about it at all.
With that said you have already pulled your fish so I guess treat away. Just go word for word on the back of the cupramine bottle. Be careful having 7 fish in a 30 gallon....if ammonia starts to rise you run the risk of killing all your fish and most types of ammo-locks cant be used in conjunction with cupramine/copper.
To keep the mandarin fed you will need to get him to eat frozen food, no way around it. Not sure how dragonites do during treatment. These dont get treated often since they dont get ich due to their slime coat. You could run the treatment and throw him back quickly and hope he doesnt starve to death by then, I doubt he would.