For studio walls, in the wall cavities, the cheapest "fluffy" insulation should be used.  A standard low density (and low GFR) insulation, will do what you need wall cavity insulation to do.

• Any sort of "blown in" loose insulation, as it can sag over time, making your isolation worse.
• Any sort of cellulose foam type insulation, it makes isolation worse
• Rigid foam insulation makes isolation worse
• Semi-Rigid insulation, like Owen Corning 703, or Rockboard. These are expensive, and not meant for wall cavities. They are effective for treating the interior of your studio as acoustic absorbers

Semi-Rigid insulation like Owen Corning 703, or Rockboard are expensive, and not meant for wall cavities. They are effective for treating the interior of your studio as acoustic absorbers

For studio walls, in the wall cavities, the cheapest "fluffy" insulation should be used.  A standard low density (and low GFR) insulation, will do what you need wall cavity insulation to do.

It will dampen resonances in the wall cavity. (Think palm muting a guitar, or moon gels on drum heads)

It will increase isolation by 3-6 decibels.

No. Any sort of cellulose foam type insulation makes isolation worse.

In some cases building code will require you to use a mineral wool type insulation, also known as "fire safe" insulation.  If this is the case then use the cheapest, lowest density mineral wool available.

Thicknesses are named with a capital R, then a number. Example R-19. Insulation comes in various thicknesses, in large rolls, or strips (called "batts").

Insulation will offer 3-6db of isolation when used in wall cavities, compared to an uninsulated wall.

Use insulation that is the thickness of the framing. A 2x4 wall frame would need 4" thick insulation.

No. You should only use enough insulation to loosely fill the wall cavity.

• Packing insulation too tightly. Insulation should be loosely packed in the wall cavity. It shouldn't create a firm or solid connection between to isolated wall frames.
• Not securing the insulation properly allowing it to sag, or fall out of wall frame.

Yes. The entire airspace can be filled, but should be loose packed. Packing too tight will make isolation worse, by forming a hard connection between wall frames.

Yes, as long as it is loosely touching, not packed tightly, or creating a hard connection (acoustic bridge) between walls.

Insulation will naturally expand slightly, and extend into the airspace between walls. This is not a problem, presuming standard size insulation, and a 1" or more airspace between walls.