Again, sorry for the redundancy, but EVERY room will benefit from/and will likely sound like shit, unless worthy bass traps are placed in the room.
Especially when the room is small in size.
It seems as if you have been doing some research. Keep on that path man.
The only way for you to realize the importance of this is to do it.
How many Youtube or any threads for that matter have ever stated that proper acoustic absorption/bass traps were not effective? There are none because it is what every room needs. Are there shortcuts? Yeah, read the 1000000 half assed attempts at trying with blankets and pillows...
Ignore most of what you read, unless it comes from a forum like this.
There are professionals who spend their time giving the real advice here.
And just so you know, if poor advice is ever given on this site, it will be shot down almost immediately by the many knowledgeable members here.
You can feel safe in knowing that anything you read on the 'stickies' here has been proven or at least 'not' false information.
Again, why the hell would you wish to record anything like a acoustic guitar in an isolation/ish room?
Treat your control/main room and forget about the isolation room until you have a reason to use it.
Sorry, but I guess I just do not understand what you are trying to achieve.
You really should describe your intentions.
I actually feel bad for not asking that of you yet...
SELF