YOu do not need something living in QT, you can throw some food in their regularly to keep the tank cycled. COntrary to popular belief, it is not fish but ammonia that keeps the tank cycled. That can come from many sources.
I personally would not keep a fish living in a QT. First, it keeps a fish, a host for parasites, constantly available (instead of leaving the tank without fish for long periods, so that the parasites would die off). Secondly it constantly subjects that fish to periods of hyposalinity, medication, etc. THis could in some sense provide a fish host to a parasite that could become immune over time to your treatments (in theory, at least). Finally, it introduces a possible competitor in the QT. QT tanks are not simply hospital tanks, they allow new fish time to acclimate, calm down, and eat before going into a main display. Having any established fish in a QT, even as "harmless" as a molly (which IMO can be quite nippy at times) is not a good plan.
However, yes, it CAN be done.