Put a switch into the ethernet port provided by my ISP. Connect Xbox-es to that switch directly.
Also connect pfSense's WAN port to that switch. Connect a second switch to pfSense's LAN port. Connect the rest of the clients to the second switch.
You need to run all the cables from all the devices in the rooms one by one to the location where these switches are.
If you have only one single cable to each room, and you use local switches in each room to split the network to multiple devices, you're in trouble.
You either lay new cables, or you buy lots of small VLAN-capable switches everywhere.
For a VLAN-ed setup, you need this:
a master (core) VLAN-capable switch at the place where the ethernet port provided by my ISP comes in.
smaller VLAN capable switches everywhere else
in each switch, create two VLAN, say VLAN 10 and VLAN 20
in the core switch, assign both VLANs as tagged, to ports going to the other smaller switches
on the other switches, assign both VLANs as tagged, to the port which connects to the core switch
in the core switch assign VLAN 10 to at least 2 ports, one for the ISP cable and one for pfSense's WAN port (yes, VLAN10 will carry the ISP's 192. 168.x.x network)
in the other switches assign VLAN 10 to the port going to XBOX.
in the core switch assign VLAN 20 to at least 1 port, and connect pfSense's LAN here (and this way VLAN 20 will carry your pfSense's LAN network)
in the other switches assign VLAN 20 to the other ports for devices which are not XBOXes.
Example of VLAN-capable small switch (gigabit, 5-ports): http://www.tp-link.com/lb/products/details/cat-41_TL-SG105E.html
Example of VLAN-capable bigger switch (same as above but 16 ports): http://www.tp-link.com/lb/products/details/cat-41_TL-SG1016DE.html
These are cheap, but smarter series are http://www.tp-link.com/lb/products/details/cat-40_TL-SG2008.html and http://www.tp-link.com/lb/products/details/cat-40_TL-SG2216.html respectively.
Good luck!