Do not try to mix MBR and GPT/UEFI. It is asking for trouble. Just recreate your Linux usb stick with GPT/UEFI support, assuming that the linux distribution supports UEFI. All current *buntu flavors do.
For Ubuntu 15.10 Unetbootin no longer works. Maybe rufus still does, I do not know. dd does work:
assuming that /dev/sdb is your usb stick. I do not think the bs parameter matters very much.
# dd if=<your iso file> of=/dev/sdb bs=1024
If the dd command scares you, you can have a look at https://help.ubuntu.com/community/Installation/FromUSBStick#mkusb_-_dd_image_of_iso_file_to_USB_device_safely