A few clarifications will help me to better understand your system. Forgive me if they have already been covered in the 15 page thread you linked too.
Is the AC shown a dedicated system for the studio or does it also serve the house? What are the sizes of the studio and control rooms?
What size is the unit? It looks like a Trane, but whatís the model?
Is there turning vanes installed at the top of the vertical duct rise? If not, there should be. The horizontal run looks like it will be internally lined rectangular metal duct, correct? Whatís that piece of floppy laying the vent stack for, a supply duct? Out of curiosity, did a professional install that vent stack? Long horizontal runs of vent pipe are tricky and can easily become a fire or monoxide hazard. Just want to make sure your code compliant and safe! (Max horizontal length= 75% of height as measured from connector at unit to the bottom of vent cap. An angle of 45 degrees or more is considered horizontal. Min slope of 2 degrees slope) What about combustion air into the mechanical room? Has that been addressed? Can you post another picture looking from the room at the AC unit? ( Looking into the RA duct prior to the Electronic Air Cleaner)
>1. For the send, I need a short height (6"-8"), but width I can go approx 24". What type of grill would I need to install for this type of area and what is the "rough in" i need to make the hole? <
Will you be installing sidewall grilles or ceiling mounted? How much CFM do have for each room? How many inlets/ outlets per room will there, or can there be?
2. Same question for the return. What is a recommended size and grill section I can use for the control room. Steel Studs are on 24" center.<
Same questions as above. Can you get your HVAC guy to purchase the grills for you?
>3. How do I decouple the duct work from the drywall or framing of the wall? Or is this even possible? <
If your using duct board I would not worry about it. Otherwise a simple canvas connection will do the trick. The photos raise another flag in my mind. What about fire dampers? If your studio will be used as a public space then you certainly have a separation of occupancy which would require fire dampers. If your going to run a business in there and have paying customers in your studio you really should check into you liability.
>4. Do I use the super duct stuff the entire way, or do I integrate it w/ some flex duct at some point? <
If your going to sidewall feed the rooms I would just keep with properly sized duct board, fewer transitions will be required.
If you can stand it, check out my rather long-winded next post in which I will explain some HVAC basics. This should give some insight to general design practices to follow.