I have built a similar project on a Beagleboard xM running Ubuntu. This could equally be a Galileo board running Linux.
I receive data from several Digi Xbee modules mounted on various Arduino+shields and then transmit the information up to Thingspeak.
I use a digi XStick plugged into the Beagleboard. I talk to the Digi X-stick using Ruby, and just treat it as a serial device.
The XStick is equivalent to the uartSbee from Seeed.
You don't actually need an Arduino to drive the XBee devices. You can use an adapter such as the Seeed or Adafruit one.
For multiple Zigbee products you need to use api mode.
You do not have to run Linux on the Galileo board - there is an excellent xbee library that can be found on the Arduino website, but not sure if it has been ported to the Galileo.
If you know what you want to do, then you don't need to to code every function. Just pick what you want to do. In my experience, the main effort is setting up the data correctly... the X-CTU utility is great here to check that things work.
A really good book for this is "Building Wireless Sensor Networks" by Faludi (O'Reilly).
There is also "The Hands on XBee LAB Manual" , this has a useful spreadsheet attached to it for setting up the XBee data stream which you can download without having to buy the book, I think.
Hope this helps, and I have understood what you are trying to do.