that's quite a few questions (and topics) but here's some starting points..
1. Isolation: As you probably know (from you're earlier studio construction) the primary ingredients to good sound isolation are mass and air space... Lay out a good double wall construction plan with multiple layers of multiple materials on each stud face... If you want to install a floating floor, there are sooooo many options. It's impossible to recommend one here as it depends on host building compliance, type of construction that is practicle, budget, load capacity, etc, etc ,etc...
Any way, build your new iso booth wall on top of the the floating floor to maximize effectiveness.. Yes you will give up some space, but sound isolation takes space!!
2. Treatments: In this size room, you would NOT be looking for a lot of reflective surfaces. We would typically design a "Medium" or natural sounding room for this application. Mostly trapped (Absorption). Don't forget to put deeper trapping in corners for broadband bass absorption... then we would use some helmholtz slat trapping on at least one or two walls... I would usually angle the framing for the trapping to vary the airspace behind the slat wall (thus de-tuning it and making it more broadband).
I'm sorry I can't get too specific as the detailing on all of this is what we do for a living.... all the information is available in many sources.
I'm just trying to make some suggestions and get the thread started. I'm sure everyone will chime in with great advice, as always!!