Late response, but:
1) Yes, any number of devices can be placed in a PAN under a single primary device. They don't technically even need to be physically near each other, just registered to the PAN. You can NOT pick and choose Matrix Attributes from any and all devices under a PAN - you pick one device to be Primary, and only that device's matrix attributes matter.
2) Commlinks provide Data Processing and Firewall, but do not "stack" with anything. Commlinks are basically smartphones except the interface is AR or VR. They are used for day to day communication and public data lookup. They are not really suitable for hacking. They can run Basic and Commlink programs but not Hacking or Rigging programs. The "Autosoft Host" Basic Program can allow an attached Agent Box to use loaded Autosofts or Skillsofts - more on this below. With a Control Rig, they can be used to rig a single vehicle or drone.
Rigger Command Consoles provide Data Processing and Firewall and are used for commanding fleets of vehicles. They are useful but not required for Riggers. They can run Basic and Rigging programs but not Hacking programs. The "Emulator" Basic Program can let the device run Commlink Programs. With special programs, they can emulate Attack or Sleaze but only for specific tests, like "Run Silent, Run Deep" adds a Sleaze rating but just for defending against Matrix Perception.
Cyberdecks pair with *Cyberjacks* and ONLY Cyberjacks. Not Commlinks, not Rigger Command Consoles. The pairing provides all four Attack, Sleaze, Data Processing, and Firewall matrix attributes. Cyberdecks combined with Cyberjacks are really the only device that can be used to hack. They can run Basic programs. Cyberdecks are also the only device that can run Hacking programs. The "Emulator" Basic Program can let the device run Commlink Programs. The "Autosoft Host" Basic Program can allow an attached Agent Box to use loaded Autosofts or Skillsofts - more on this below. With a Control Rig, they can be used to rig a single vehicle or drone.
Agents are semi-smart programs that can do some hacking on their own. They can only be directly loaded into Cyberdecks, but can also be loaded into Agent Boxes and be attached to any device. They are, however, still limited to actions the device they are attached to, so an Agent Box linked to a Commlink is fairly useless except as possibly a personal secretary.
3) Currently there are no limits on number of Program Carriers. So go nuts. Note that they do not change what types of programs their attached device can run.
-k