You are entering into post XP networking Hell if you want to set custom
permissions for sharing particular folders/files.
The permission settings are arcane and non-intuitive. Even worse, the
settings you laboriously create may work for a time and then no longer work.
Microsoft will deny that instability exists but I doubt there is a single
user who has not experienced it.
Microsoft wants to force you to use the "Public" folders for this purpose.
If that suits you then what you propose will be easier to accomplish. If you
are not familiar with networking I recommend you start there.
You will have to set up programs on each computer to save files, like
documents and images, to Public folders.
If you use the firewall built into Vista/Win7, and there is absolutely no
reason for most people to use anything else despite what you may have read,
then there should be no problems with the firewall.