Insulating is good, but the hard lab data doesn't always seem logical. The best in-wall or in-ceiling insulation for acoustics is the cheapest fiberglass you can get at Home Depot, Lowes, etc. Folks can argue all day, as this seems illogical. However that's the lab data.
The good news is that it's also the cheapest insulation.
Regarding the drywall, as I already mentioned I'm in the standard 5/8" drywall camp. I don't have water on the floor (I have alarms if that happens) and never had mold. Just keep it off the floor so there's no water conduction. Fill that gap with caulk.