Even if you don't want to prophylactically medicate fish which have already been quarantined, I would still observe in QT for one month. This allows enough time for most diseases to present themselves, and as an added bonus you get to strengthen & condition your new specimen before he joins your other fish in the DT. Who will probably be less than thrilled with the arrival of another fish in their territory.
If you don't like the bare bottom/rockless QT concept, then setup a mini-version of your DT to observe new fish. Just be sure to have another tank on hand (plus all equipment) to medicate in, if the need arises.
The successful QT I've done has been just as you describe. Healthy fish, in a 10G bare bottom tank, no mediations, just observed them for 4 weeks. I did eventually put a little decoration in it, so it wasn't so bare. But again the fish were healthy already.
I have two 10G that I can use for this purpose and won't be used otherwise. Just really bummed about the fish I just lost. Thanks for all the great info and suggestions