i QT'd my carpenter's wrasse without live rock or sand, basically the same thing as a mccosker's from what i understand.
i just fed him a couple of times a day, to make sure he was getting enough. he didn't seem too worried about picking at anything, and was in QT like this for about 12 weeks. i use a fully cycled QT as well, as more of an observation tank. if i need to medicate, i would remove the fish to a dedicated hospital tank.
sadly i lost him about a week ago. i think he jumped out. my DT has good coverage on the top, but due to some hang on the back stuff, it is not perfect, and i think he was able to slip through. so make sure you're covered tight!
i'm not sure about any of the specifics with the yellow fin, but the carpenter's/mccosker's are awesome fish. enjoy!