Have You managed to install ClearOS to the usb stick?
I have installed several ClearOS instances to SSD drives, so if the USB Stick install will not go ok, and if You are after energy savings and reliability (less mechanical parts), You can always try the small 32gb ssd way.
I think that the 2.6.18-308 headers are already available / anyhow just keep a copy of the grub.conf file and whatever changes you make, best to comment out the previous versions instead of deleting.
